I-123 is a radionuclide frequently used in nuclear medicine imaging. The image formed by the 159 keV photopeak includes a considerable scatter component due to high energy gamma- ray emission. In order to evaluate the fraction of scattered photons, a Monte Carlo simulation of a scintillation camera used for 123I imaging was undertaken. The Monte Carlo code consists of two modules, the HEXAGON code modelled the collimator with a complex hexagonal geometry and the NAI code modelled the NaI detector system including the back compartment. The simulation was carried out for various types of collimators under two separate conditions of the source locations in air and in water. Energy spectra of 123I for every pixel ( matrix size = 256 x 256) were obtained by separating the unscattered from the scattered and the penetrated photons. The calculated energy spectra ( cps MBq(-1) keV(-1)) agreed with the measured spectra with similar to 20% deviations for three different collimators. The difference of the sensitivities ( cps MBq(-1)) for the window of 143-175 keV was less than 10% between the simulation and the experiment. The partial sensitivities for the scattered and the unscattered components were obtained. The simulated fraction of the unscattered photons to the total photons were 0.46 for LEHR, 0.54 for LEGP and 0.90 for MEGP for the ` in air' set- up, and 0.35, 0.40 and 0.68 for the ` in water' set- up, respectively. The Monte Carlo simulation presented in this work enabled us to investigate the design of a new collimator optimum for 123I scintigraphy.