The purpose of this study was to construct the assertiveness scale for children (ASC). In the first study, 94 fifth graders answered to the original ASC, the social desirability scale for children (SDSC), and the agreeableness scale of the Motoaki-Guilford personality test (AG). Original ASC included 36 items covered 6 domains of assertiveness: (1) defense of right,(2) turn down of request,(3) expression of different opinion,(4) expression of personal limitation,(5) request of other's help, and (6) expression of positive feeling. Correlation coefficients were calculated and 18 items that correlated significantly with SDSC score and/or AG score were excluded from ASC. Finally, ASC contained 18 items significantly correlating with neither SDSC score nor AG score. In the second study, 238 fourth, fifth, and sixth graders answered to ASC and to 4 peer-nomination items that asked assertiveness of peers. Cronbach's α was, 77 and ASC score correlated with all peer-nomination items. The data from the other sample showed that test-retest reliability (3 months) was, 72. It was concluded that ASC had both reliability and validity.