Nitrification is one of the most active biological processes in the soils receiving ammonium nitrogen. The rate of this process is under the influence of several factors and their interactions. In this study, the effects of ammonium concentration and moisture content on the extent of nitrification in two soil samples named A (Loam) and B (Clay loam), which had been taken, respectively, from Marand and Ahar areas, were investigated. A two-week factorial incubation experiment (25±0.5°C) was conducted in a completely randomized design with three replications. Factors were urea nitrogen at five levels (0, 50, 100, 200 and 400 mg N kg^-1), moisture content at three levels (0.55FC-0.60FC, 0.75FC-0.80FC and 0.95FC-FC) and two soil types (A and B). At the end of the experiment, concentrations of ammonium and nitrate as well as the values of pH and EC were determined. Based on the results, average nitrification at 0.55FC-0.60FC was 22 percent lower than that at 0.95FC-FC and no significant difference was observed between 0.75FC-0.80FC and 0.95FC-FC. Nitrification at the treatment of 400 mg N kg^-1and 0.55FC-0.60FC was decreased considerably and 25 percent of the added ammonium was accumulated. The average ammonium concentrations did not significantly vary among the levels of 50, 100 and 200 mg N kg^-1, but these concentrations were significantly lower than those of 400 mg N kg^-1. Moreover, EC and pH values of the soils were significantly increased and decreased in response to the nitrification (0.54 dS m^-1 and 0.59 at the application level of 200 mg N kg^-1, respectively). On average, the results showed higher nitrification (40.3 mg N kg^-1) in the soil A (Loam texture) than the soil B (Clay loam).