Field evaluations were conducted to determine the efficacy of Orius strigicollis for biological control of Thrips palmi with different initial density in plastic houses. O. strigicollis released three times after 0, 5, 30, 100 thrips were inoculated per 30 cucumber plants. Three hundred and sixty predators were released in each plot. After inoculation of 5 T. palmi per 30 plants in each plot in plastic houses, T. palmi population increased rapidly with 16.3 individuals per plant in the fourth week, and O. strigicollis could not suppress T. palmi population because O. strigicollis population did not build up quickly. In the plot of 30 thrips inoculation, the density of O. strigicollis increased to 2.1 individuals per plant in the third week and 9.8 individuals per plant in the sixth week. The density of T. palmi increased to 33.8 individuals per plant in the sixth week. In the 100 thrips inoculation plot, the density of O. strigicollis increased to 9 individuals per plant in sixth week, which was similar to that in the 30 thrips inoculation plot. But the density of T. palmi increased rapidly from 2.9 to 384.7 individuals per plant in the sixth week. So, control of T. palmi by O. strigicollis did not appear. In autumn culture, in the 5 thrips inoculation plot, the density of O. strigicollis was so low that it could not suppress thrips density. In the 100 thrips inoculation plot, the density of T. palmi was so high that O. strigicollis could not suppress T. palmi density. In the 30 thrips inoculation plot, the density of O. strigicollis was 4.8 individuals per plant in the sixth week and the thrips population was suppressed. But O. strigicollis could not suppress the thrips population after sixth week because of low temperature and short photoperiod. These results indicate that control of T. palmi using O. strigicollis may be possible on cucumber in the spring culture.