The use of building materials containing naturally occurring radionuclides,such as 40K,238U and 232Th and their progeny,could lead to external exposures of the residents of such buildings.So it is important to assess the dose imposed to inhabitant according to radioactivities of the building materials,which can be measured simply and precisely.In this paper,a set of models are constructed to calculate the specific effective dose rates(the effective dose rate per Bq kg-1 of 40K,238U series,and 232Th series) imposed to residents by building materials with MCNPX code.Effect on the dose assessment by chemical composition,position concerned in the room and thickness as well as density of the building materials is analyzed.Results show that there is no significant difference between those values for building materials with different chemical composition or for positions concerned inside the room,while the effect caused by surface density(equal to thickness×density) can not be ignored.In order to facilitate more precise assessment of indoor external dose due to gamma-emitting radionuclides in building materials,three regressive expressions are proposed and validated by measured data to calculate specific effective rate for 40K,238U series and 232Th series,respectively.