MCNPX computer code based on Monte Carlo method is used to design a computer model for an assembly of high temperature testing reactor (HTTR). Two models are used in the analysis , homogeneous and heterogeneous models. The reactor uses TRISO fuel , Graphite moderator and helium coolant. The multiplication factor of the assembly is determined as function of fuel burnup and operation time. Axial power mapping distributions are evaluated. Time evolution of actinides ( U235 and Pu239 ) are calculated as a result of fuel burnup. Fuel and moderator temperature coefficient of reactivity are determined as a function of operating temperature. The effect of Helium coolant losses on reactor criticality is evaluated bu assuming reduction of helium density to one percent of its nominal density. The results indicated that homogenous model results are in good agreement with heterogeneous models with average difference of approximately 5 %. This enable homogeneous model to be used in full reactor core simulations.