The synthesis of a new series of 1β‐methylcarbapenems having a 1,3‐diazabicyclo[3.3.0]octan‐4‐one moiety is described. Their in vitro antibacterialactivities against both Gram‐positive and Gram‐negative bacteria are tested and the effect of substituent on the bicyclic ring was investigated. A particular compound (11h) having aminoethyl group showed the most potent antibacterialactivity.