The purpose of this study was to investigate the effects of 8-week medicine ball training on muscular strengths of the upper body. Fifteen baseball players were divided into medicine ball training group (MB group, n=7) and control group (group C, n=8). All subjects maintained the regular baseball training programs during the experimental period. The subjects of MB group had additional medicine ball training 3 times per week. The maximal muscle strength of push-up, pull-down, back extension, abdomen curl, as well as left and right twist motions of subjects were tested before and after training. Independent t-tests and paired t-tests were used to compare the variables between and within groups. There were no significant differences in variables between two groups before training. After training, the maximal muscle strengths of abdomen curl and right twist motion in MB group were 30.3±3.6 kg and 45.5±10.2 kg. The values were significantly higher than those in Group C (21.5±3.6 kg and 27.8±11.0 kg, respectively). There were no differences before and after training in Group C. However, the maximal muscle strengths of pull down, back extension, abdomen curl, as well as left and right twist motions in MB group were significantly higher than those in pre- training (p<.05). During the experimental period, the muscle strengths of back extension, abdomen curl, left twist and right twist motions increased 20.64%, 58.66%, 69.38% and 25.65%, respectively. It is concluded that medicine ball training program of this study is beneficial to the muscle strengths of the upper body. Medicine ball training may be applied to those who have lower abdominal muscle strengths and are without the equipments of weight training for enhancing the muscular strengths of the upper body.