Abstract: In the present study, a new detector collimator has been designed to detect the buried anti-personal (AP) landmines for use in gamma-ray backscattering device, with Monte Carlo method. The new detector collimator is made by lead and it has the cylindrical shape with 11.2 mm in height, 12.6 mm in inner radius, 13.3 mm in outer radius and 0.7 mm in thickness of the shielding layers around detector. The mass of the designed detector collimator is also 7.24 g.