ET is a nuclear medicine imaging method which is used to produce 3D images of human body. In this method, imaging requires a source of positron. This positron interacts with an electron to produce two annihilation gammas at an angle of 180 degrees. Imaging is based on identification of the two photons, which energy of each is 511 keV, at the same time by two detectors at the angle of 180 degrees. In order to obtain a proper quality of the images, the detectors should have good efficiency for the 511keV photons. The aim of this work is to investigate the efficiency of PET detectors using MCNP code. The geometry used in this work is: 64 cube detectors with dimensions of 4 mm 3 are located on a cylinder with a radius of 40cm and a length of 2cm. An isotropic point source emitting positron is located at coordinate origin at cancerous tissue. The tissue is elliptical shape with a small radius of 2.5cm and a large radius of 6cm inside the phantom of the body(elliptical cylinder) with a length of 160cm and a large radius of 17.5 and small radius of 10cm. Detectors used in this study are scintillator detectors that are: GSO, BGO, LSO, LYSO,YSO, BaF2 and NaI(Tl). We considered detectors in the same dimensions and different thicknesses of 10, 20 and 30 mm on a cylinder with radius 40cm and calculated the efficiency by Monte Carlo simulation. The highest efficiencies for all three thicknesses were found for the detectors BGO, LYSO and LSO. Therefore, considering the density, energy resolution and efficiency for different thicknesses, the most suitable detector for PET is LSO.