The effect of inorganic fertilizer (NPK) on earthworm population was studied for two years (2002-2004) under different plantations at Sakawrtuichhun (SKT) and Pachhunga University College (PUC) agroforestry sites in Mizoram, north-eastern state of India. Five species of earthworms, viz. Drawida sp., Metaphire houletti (Perrier), Perionyx excavatus (Perrier), Perionyx macintoshi (Stephenson) and Eutyphoeus mizoramensis (Julka et al.) under three families were identified in the study sites. Detailed study was carried out under different soil strata and on different age group of earthworms in plantations like maize+Leucena leucocephala (ML) and pine apple+citrus+Leucena leucocephala (PCL). Doses of inorganic fertilizer treatment were given as per state agriculture department recommendation. Earthworms were collected from five random sampling sites at monthly intervals by hand sorting method in each replicate. The most versatile species was found to be Drawida species. An epigeic species, P. excavatus showed significant population variation between NPK treatment and control in both study sites. The one-way ANOVA showed positive impacts of NPK fertilizer treatment on earthworm population. Four species of earthworms excluding the anecic E. mizoramensis showed a significant variation between the two treatments in PCL plantations of PUC. The uppermost stratum (0-10 cm) seemed to be the most affected by fertilizer treatment among the three strata. The Juvenile and immature worms were found to be most influenced by NPK application. The impact of NPK application on earthworm population was greater in PCL plantation compared to ML plantation.