The oligophagous pest Plutella xylostella (L.) is a major crucifer pest in Tehran Province, Iran. Hexaflumuron is an insect growth regulator insecticide with good effects on immature insect stages. The objective of this study was to investigate the effects of two sublethal concentrations (LC10 and LC25) of hexaflumuron on some biological parameters of P. xylostella larvae, such as birth rate (b), death rate (d), finite rate of increase (λ), generation time (T), sex ratio, pupation rate, and adult emergence. Results showed that hexaflumuron decreased the total number of eggs and oviposition and post-oviposition periods, pupation, and adult emergence in the treated generation, b, and λ. Hexaflumuron also increased T, d, and the pre-oviposition period. However, sex ratio, percentage of pupation, and adult emergence in the offspring generation were not affected by hexaflumuron. Overall, these results indicated that sublethal concentrations of hexaflumuron can affect the biological parameters of P. xylostella.