Abstract: This study investigated the effects of larval density on the population dynamics of diamondback moth (Plutella xylostella). It analyzed the growth, development and reproduction of P. xylostella of five larval densities (1 larvae·dish^-1, 10 larvae·dish^-1, 20 larvae·dish^-1, 30 larvae·dish^-1 and 40 larvae·dish^-1) under laboratory conditions of (22±1) ℃, 70%~80% RH and 12L∶12D photoperiod. The results showed that immature duration, survival rate, pupal weight and adult reproduction of P. xylostella were significantly affected by larval density. The time of development of larva and pupa was longest at the density of 10 larvae per dish and significantly prolonged with increasing larval density. Survival rate from larva to pupa was highest at the density of 10 and 20 larvae per dish, which was significantly higher than those at the density of 1 and 40 larvae per dish. Pupal weight was heaviest at the density of one larvae per dish, which significantly dropped with increasing larval density. Abnormalities of moth adults were significantly different at different larval densities. Abnormality was lowest at the density of 10 larvae per dish, which significantly increased with increasing larval density. Total fecundity per female was highest at the density of one larva per dish, which also gradually declined with increasing larval density. Female and male longevities were highest at the densities of 40 and 10 larvae per dish respectively. It significantly shortened when the density was higher or lower. Life table analysis suggested that larval density significantly affected population growth index. It was highest at the density of 10 larvae per dish and dropped for both higher and lower densities. It was therefore concluded that larval density significantly influenced population growth of P. xylostella.