The k0-based internal monostandard INAA method (IM-NAA) has been investigated for the determination of elemental concentrations in non-standard geometry samples at the Dalat research reactor (DRR). The experiments were implemented at the rotary specimen rack of the DRR which the thermal neutron flux was (3.61 ± 0.11) × 1012 n cm−2 s−1. In this work, the concentration analysis of Montana II Soil (SRM 2711a) and Brick Clay (SRM 679) have been used to validate the method. The elemental concentrations of Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Rb, Sb, Sm, Ta, Th, Yb, Zn have been analysed and to be in good agreement with certified values. The IM-NAA method has been applied to determine elemental concentrations in clay bricks and ceramics at the Cat Tien archaeological site.