This study investigated the effect of SWCNTs' length on their antimicrobial activity to bacterial cells in suspensions. Three different lengths of SWCNTs (< 1 mu m, 1-5 mu m, and similar to 5 mu m) were tested. At same weight concentration, longer SWCNTs exhibited stronger antimicrobial activity. The fluorescence and SEM images revealed that the longer SWCNTs aggregated with bacterial cells more effectively, whereas short length SWCNTs tended to aggregate themselves without involving many bacterial cells. Moreover, longer SWCNTs exhibited more pronounced concentration-dependence and treatment time-dependence on their antimicrobial activity.