Current geoid modelling procedures assume an average density of 2.67 g/cm3, which appears to be inadequate for geoid mode1ling at the centimetre level of accuracy. We have produced the topographic mass density map of Canada based on the digital geological map of the country and in-situ rock density values from various geological studies and actual rock samples. Representative density values of the rock types in Canada vary by as much as 13% when compared to the mean value of 2.67 g/cm3.In the Skeena region of British Columbia, Canada, the effect of the variable density on the terrain corrections to gravity ranges nearly three mGal, while in the rolling hi1ls of Northern New Brunswick, Canada, the range is about 5mGal. Consequently,the effect on the geoid ranges nearly 10 centimetres and seven millimetres respectively for the two test areas.