Steady mixed convection heat transfer in Sisko fluid over a vertically stretching surface is considered when imposed and buoyancy induced flows are in the same and opposite directions. The flow is induced due to a nonlinear stretching sheet with constant temperature distribution. Viscous dissipation effects are also taken into account. The governing partial differential equations under the framework of a Boussinesq approximation are transformed into ordinary differential equations using a suitable transformation. These equations are solved numerically using an implicit finite difference method for the following parameters: buoyancy parameter (-3.0 <= lambda <= 2.0), power-law index (0.28 <= n <= 1.9), material parameter of Sisko fluid (0.2 <= A <= 1.9), and Eckert number (0.2 <= Ec <= 0.9). In particular, the effects of these dimensionless parameters on the velocity and temperature fields, the skin friction coefficient, and the local Nusselt numbers are displayed graphically. Numerical results are validated by comparison with analytic results as well as previously published results as a special case of the problem. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.