Natural convection of high Prandtl number fluids with temperature-dependent viscosity in a vertical slot with aspect ratio 15 is studied numerically. A four-parameter fit For the variable viscosity is adopted. The flow patterns and the critical Grashof numbers predicted are in good agreement with earlier experimental results of Chen and Thangam [J. Fluid Mech. 161, 161-173 (1985)]. The critical Grashof number is lower than the corresponding value obtained for the constant viscosity fluid, and it varies almost inversely with the Prandtl number. The viscosity stratification and the large vertical temperature gradient in the lower region of the slot cause the secondary cells to appear in the upper region of the central core at onset. The heat transfer rate is slightly smaller than that in constant property case as the viscosity variation becomes large.