APOBEC3F and APOBEC3G cytidine deaminases potently inhibit human immunodeficiency virus type 1 (HIV-1) replication by enzymatically inserting G-to-A mutations in viral DNA and/or impairing viral reverse transcription independently of their deaminase activity. Through experimental and mathematical investigation, here we quantitatively demonstrate that 99.3% of the antiviral effect of APOBEC3G is dependent on its deaminase activity, whereas 30.2% of the antiviral effect of APOBEC3F is attributed to deaminase-independent ability. This is the first report quantitatively elucidating how APOBEC3F and APOBEC3G differ in their anti-HIV-1 modes.