For the two-dimensional mixed convection between vertical parallel plates kept at uniform temperatures, linear stability analysis is performed numerically to obtain the marginal curve in Re-Gr/Re plane. Riccati method is used to solve the linearized stability equation, and the result shows good agreement with the previous result at Reynolds numbers ranging from 0 to 100. For 22505500, $\alpha_c$ decreases as Reynolds number increases. At Re = 13175 and Gr/Re = 34.21, the marginal curve of interactive instability and that of shear instability cross each other with different $\alpha_c$ and $Cr_c$.
