Experimental facilities like HTR-10, HTTR, and ASTRA serve as the source of information for the currently designed high temperature gas-cooled nuclear reactors. It is also desired to verify the existing codes against the data obtained in such facilities. In this study. first criticality calculations of a pebble bed gas-cooled reactor, HTR-10, is performed with MCNP-4B, a code system for Monte Carlo particle transport simulation. HTR-10 has rather unique characteristics in terms of the randomness in geometry as in the case of all pebble bed reactors. The geometrical model of the full reactor is obtained by using lattice and universe facilities provided by MCNP. Modeling details are discussed with necessary simplifications. Results obtained by Monte Carlo simulations are compared with available data. It is observed that Monte Carlo simulations yield sufficiently accurate results in terms of initial criticality of the HTR-10 reactor. (C) 2003 Elsevier Science B.V. All rights reserved.