Field experiments were conducted from 1999 to 2000 to assess the effect of nitrogen fertilization levels (0.225 and 450 kg.hm-2) on the population dynamics of cotton bollworm (Helicoverpa armigera) and cotton aphid (Aphid gossypii), and on the numbers of cotton buds and bolls in North China. The population densities of cotton bollworm and cotton aphid increased with increasing nitrogen fertilization during 1999 to 2000, but the differences between treatments were not significant (P > 0.05). There were significant differences of cotton bollworm populations and no significant differences of cotton aphid populations between years. Nitrogen fertilization at flower-bud stage decreased the loss of cotton buds and flowers and increased the number of effective bolls and cotton yield, but the decrement and increment did not get to significant level (P < 0.05). No obvious correlations of nitrogen levels with main pest abundance and cotton production were observed.