Abstract: On the basis of same heat flow between hot surface and col surface, the concept of effective thermal conductivity is adapted to solve thos problems which have combined convection and conduction in enclosure. The relations of effective thermal conductivity with the shape of cylindrical enclosure Re number and Gr number arc calculated, and all the results are explained physically. Those results are available to solving the problems such as metal solidification and crystal crystallization etc. The comparison is made between the calculating results and experimental data of GaAs crystal growth, and it is founc that they are agreement.