Three irradiation positions of Apsara reactor were characterized by determining epithermal neutron spectrum shape factor (α) and sub-cadmium to epithermal neutron flux ratio (f) for k_o-based InternalMonostandard Neutron Activation Analysis (IM-NAA). The values were determined by cadmium ratio method using six neutron flux monitors (In, Au, Co, Mo, Zn and Zr). The α value was also determined by cadmium ratio method using an Aumonitor. The values of two irradiation positions are found to be positive, which indicated softened epithermal neutron spectra whereas α value of the third one is negative indicating hardened neutron spectrum. IAEA reference material, RM SL-1 (Lake Sediment) was analyzed by IM-NAA using experimentally determined f and  α values of all three irradiation positions. A total of 16 elemental concentrations were determined using Sc as the internal monostandard and the values are found to be in good agreement with respect to the certified or information values. As an application of IM-NAA, two ceramic archaeological samples along with one soil sample were analyzed using Sc as the monostandard.