Abstract: This field research was carried out at Udon Thani Province, Northeast Thailand with the use of Nampong soil series (Ustoxic Ouartzipsamment) during 1998 to investigate the effect of earthworm (Pheretema sp.) activities on the changes in chemical properties of Nampong soil series at different soil depths in different eco-systems. Soil samples were taken once at the soil depth of 0-10, 10-20, 20-30 em from locations with and without earthworm activities of the four eco-systems for the determinations of EC, pH, OM, available P, exchangeable K, Ca, and Mg. The results showed that earthworm activities, in most cases, have improved soil chemical properties at different depths in all locations studied and it was found that soil electrical conductivity (EC), organic matter (OM), available phosphorous (P), exchangeable potassium (K), calcium (Ca) and magnesium (Mg) values of tamarind orchard farm were much greater than those of other studied sites both with and without earthworm activities.