One of the major component in neutron radiography is a collimator that is used to collimate the neutron in parallel beam with less gamma ray contamination and high thermal neutron flux. The collimator consists of seven components and the interest component is an aperture as it is used to prevent the thermal neutron from entering the beam except through the center hole. In this study, the collimator design was taken from radial beam port at NR facilities at ANM with the collimation ratio is 46.4. In order to increase the collimation ratio, optimization of the aperture component has been done on four different material and 1-5 cm diameter parameters. The optimization of apertures shows that the cadmium with 1 cm diameter yields the thermal neutron flux at the collimator inlet and outlet with 1.78 x103 n cm-2 s-1 and 5.90 x102 n cm-2 s-1 while the gamma ray contamination was 10.7 μSv hr-1. The optimization succeed to produce high L/D ratio however the thermal flux was low and the gamma contamination was higher that original design but satisfied the ICRP 74 condition for radiation worker.