Abstract: Effects of host size on oviposition and development of Campoletis chlorideae Uchida, the endoparasitoid of Helicoverpa armigera (Hübner) were studied in laboratory. Host size had significant influence on oviposition of the female parasitoid, and the parasitization rate decreased as the host size became larger. 92% of the parasitized host larvae were less than 18mg in weight, and the host larvae over 18mg were difficult to be parasitized. In the laboratory condition, the vulnerable period of H. armigera larvae for parasitism was 136h (ca. 6d) after hatching. Linear regression and correlation procedures were used to quantify the relationships among parameters. The host weight at time of parasitizing was positively related with approximate larval weight of the parasitoid, and approximate larval weight of the parasitoid was positively related with adult weight of the parasitoid; in female parasitoids relationship was more significant. The larger parasitoid female adult had a longer longevity in certain range except overweighted female adult; but host weight at parasitism had no effect on total development time of the parasitoid.