Fitness associated with pesticide resistance was assessed in twospotted spider mites, Tetranychus urticae Koch, using life table analysis. Two strains resistant to fenpyroximate (FR) and pyridaben (PR), and one susceptible strain (S) were included in this study. Two reciprocal crosses (F $R_{♀}$x $S_{♂}$ and P $R_{♀}$x $S_{♂}$) were established by conventional crossing procedure. Intrinsic rate of natural increase (${gamma}$$_{m}$) was used as a comparative parameter of reproductive fitness among 3 parent strains (FR, PR, and S) and two reciprocal crosses. Sensitivity analysis was performed to evaluate significances of biological factors contributing to ${gamma}$$_{m}$. The ${gamma}$$_{m}$ values were not significantly different from each other, with an exception of F $R_{♀}$x $S_{♂}$ cross and ranged from 0.250 to 0.274 per day. Sensitivity analysis revealed that birth rate (b) and mortality ($mu$) effects could account for the effect on ${gamma}$$_{m}$ in the two resistant strains. Contributions of b and $mu$ to ${gamma}$$_{m}$ was markedly different in two resistant strains; high contribution in b and low in $mu$. Age at the first reproduction (t) and age at the last reproduction ( $t_{Omega}$) made roughly equal contributions to ${gamma}$$_{m}$ between the strains and the crosses. This study clearly showed that how ${gamma}$$_{m}$ was affected by various biological factors and provided a basic analytical tool for pesticide resistant management.nt management.