Pharmacological studies have suggested that serotonin 5-HT_2C receptor is involved in locomotor activity, anxiety, and fear memory. However, the results of locomotor activity and anxiety in 5-HT_2C receptor knockout mice are mixed, and the effects of 5-HT_2C receptor knockout on fear memory have not yet been addressed. In the present study, we reconciled these inconsistent results by analyzing behavioral data in details. We revealed that the higher locomotor activity in 5-HT_2C receptor knockout mice is observed only in the late phase of the test. Moreover, we found that 5-HT_2C receptor knockout mice display a hesitating attitude, staying in the center area and risk assessment behavior, in the elevated plus maze test. This phenotype might explain the inconsistency of previous studies. In the contextual fear conditioning test, 5-HT_2C receptor knockout mice tended to show rapid within-session extinction of fear, but not between-session extinction, compared to the wild type mice.