It is essential and pressing to improve rebobility of Boolean operations in geometric modelling systems. The Boolean operations in commercial systems now available are not reliable enough because of numerical calculation errors. Numerical calculation errors make us unable to determine set membership classifications exactly. Some classifications are ambiguous because they are dependent on adopted computing tolerance. Thereby interrelated classifications may conflict with each other and it is very difficult to get correct results of Boolean operations in many cases.We introduce a reliable Boolean opration algorithm which solves the conflicts among interrelated classifcations before implementation of Boolean operation by use of reasoning technique.The algorithm has replaced the modelling module of the geometric modelling system GEMS 2.0 to forma new system CEMS 2.1 on Apollo and Sun workstations.According totests and compared with Geomod 3.9 and I/EMS,CEMS 2.1 is much more reliable.