The purpose of this study is to analyze and compare journal citation data from Journal Citation Reports (JCR), and focus on three subjects, i.e., biology, medical science, agriculture and food science, on life science. The journal samples were drawn from the core journal list of Guide to Information Sources and "JCR science edition" on the Web 2002 in biology (327 records), medical science (129 records), agriculture and food science (130 records). The source items and five types of citation data, including citation counts, impact factor, immediacy index, citing half-life and cited half-life, are examined, and the correlation between each of the fifteen pairs of citation data is determined based on the Pearson correlation tests. The Fisher's Z-transform is employed to test the significant difference between the Pearson correlation coefficient for each pair of citation data of these three subject areas. Moreover, the T-test is used to determine the probability that the difference in the means of each citation data being observed is significantly different from zero. The results of this study reveal that: Most of the life science journals are issued monthly, bimonthly, or quarterly; the publication frequency of biological journals is various. The journals that published 1 to 99 papers are the largest group; whereas journals that published 100 to 299 papers belong to the second group. Most of the medical journals publish 300 to 1,000 articles yearly. Most of the journals are cited 1,000 to 4,999 times; medical journals are cited more than other subjects, the cited times aggregated between 10,000 and 49,999. 0.001 to 0.999 and 1.000 to 1.999 are two largest impact factor categories. Generally, biological and medical journals have higher impact factor. The immediacy index is mostly concentrated on 0.001 to 0.199. In average, immediacy index of biological journals is greater than medical journals and medical journal is greater than agriculture and food science. Citing half-life is focused on 5 to 7 years, most of the agriculture and food science journals have citing half-life more than 10 years. Most of the biological (4 to 7.9 years) and medical (7 to 7.9 years) journals obviously have shorter cited half-life than that of agriculture and food science journals (>10 years). Journals with higher impact factor may also receive higher immediacy index. The more papers published yearly the more citations the journal received. The correlations between source items and citation counts, between citation counts and impact factor and between citation counts and immediacy index are positive. Biological and medical journals have middle negative correlation between impact factor and citing half-life. In agriculture and food science journals, there is lower negative correlation. Journals of biological and medical fields reveal middle positive correlation between citing and cited half-life. The lowest correlation is between source items and impact factor. Medical and agriculture and food science journals have lower positive correlation. For journals of three subjects, there is significant difference of the Pearson correlation coefficient as the following pair of citation data: source items and citation counts, citation counts and impact factor, and impact factor and immediacy index; there are no significant difference of the Pearson correlation coefficient for source items and immediacy index and for citation counts and cited half-life. Impact factor has the significant mean difference in three disciplines, followed by immediacy index and citing half-life. The mean difference between biological and agriculture and food science journals is significant. The distinction of mean difference between journal citation's data is influenced by cited half-life and citing half-life, especially between biological and medical journals. There is no difference between the mean difference in source item and impact factor.