The biological parameters of Tetranychus turkestani (Ugarov et Nikolskii) and Tetranychus truncatus (Ehara) on cotton and corn were studied in the laboratory at 28±1°C, 70±10% RH and 16L: 8D photoperiod in 2011 and 2012. The differences of the two spider mite species on the same host plant and the effect of different host plants on the same spider mite species were both compared. The results of two spider mite species on the same host plant indicated that the r_m values of T. truncatus were higher than those of T. turkestani whether they were reared on cotton or corn. The other results of the effect of different host plants on the same spider mites indicated that T. turkestani achieved higher r_m on cotton (0.18, 0.17) than on corn (0.14, 0.15) in both 2011 and 2012, respectively. However, the r_m values obtained on cotton for T. truncatus (0.19, 0.19) were smaller than those obtained on corn (0.21, 0.21) in both years. This study showed that the fecundity of T. truncatus is greater than that of T. turkestani on the same plant species. Moreover, corn is the better host plant for T. truncatus, whereas cotton is more suitable for T. turkestani.