Seven-days old larvae of Ephestia calidella were gamma irradiated with doses from 25-300 Gy. Irradiation reduced rate of pupation and adult emergence. Also it increased adult's malformation and shortened their life span. The lethal dose for larvae was 300 Gy. Adult fecundity and fertility were adversely affected by the gamma irradiation especially at 100 Gy. Irradiation of 7-day old larvae (2n^d instar) with this dose caused severe biochemical abnormalities which extended to 14 and 18-day old larvae (4^th and 5^th or full grown larval instar, respectively). Lactate dehydrogenase (LDH), protease, cartohydrases and main metabolites, total proteins and total carbohydrates were significantly inhibited. On the contrary, alanine aminotransferase (NAT) and aspartate aminotransferase (ASAT) were elevated after treatment. The possible explanation was discussed and it was concluded that irradiation of this pest led to severe maaomolecilar changes which could affect general biology.