Female injecting drug users who are sex workers (IDUFSWs) is a strategic "bridge population" for HIV transmission. Goals of the study were to investigate condom use behaviors during commercial sex among IDUFSWs using the Theory of Planned Behavior (TPB), and to investigate moderating effects that modify the strength of associations between the TPB-related variables and inconsistent condom use during commercial sex. A total of 281 non-institutionalized IDUFSWs were recruited using snowball sampling method. Anonymous face-to-face interviews were administered by trained doctors. The results showed that the prevalence of inconsistent condom use during commercial sex in the last six months was 64%. After adjusting for some significant background variables (e.g. main venue of sex work), all associations between the five TPB-related variables and the studied condom use variable were statistically significant (Odds Ratio (OR) = 0.43-0.68, p<0.001). In the hierarchical nested models, three background variables (age, venue of sex work, and ever used HIV-related interventions) entered in the first step (-2LL = 294.98, p<0.001) and the Social Norm Scale, the Perceived Behavioral Control Scale and the Behavioral Intention Scale were selected by the second step (OR = 0.67 - 0.72, p<0.01; -2LL = 160.99, p<0.001). Significant moderating effects between some TPB-related variables (Positive Condom use Attitude Scale and Behavioral Intention Scale) and duration of sex work and duration of drug use were also reported. The results highlighted the potential of using the TPB to better understand condom use behaviors in IDUFSWs in China. Theory-based research and intervention work should be developed in China in the future.