The main issues of this study were to state principles of statistical data handling under SAS system. The principles may prevent researchers to get the results of "garbage in, garbage out" in data processing. For main issues were as follows: First, principles of statistical data hand-ling: automicity principle, appropriate data principle, data control principle, data manipulation principle, default principle, data sensibility principle and save output principle. Second, information provided by SAS LOG window including notes, warning and error. Error message such as: variable not tound, syntax error, invalid data error and execution-time error. Third, methods of data screening: PROC MEANS, PROC FREQ, PROC PRINT, PROC UNIVARIATE and PROC PLOT. Fourth, methods of working with large data set:drop all unnecessary variables, use a DROP (or KEEP) option on SET statement, do two-level sort, using a CLASS statement instead of a BY statement with PROC MEANS, use the WHERE statement instead of an IF statement, use ELSE IF instead of multiple IF statements, use PROC APPEND to add new data to a large file and use OPTIONS statement properly.