This paper presents a combination of mixed integer hydropower models and mixed integer thermal power models for short term hydrothermal power generation planning. The integer constraints in the hydro system derive from the start-up costs of hydro units and the fact that hydropower plants only are operated in finite number of intervals. The integer constraints in the thermal system derive from the start-up costs of thermal power plant units and the minimum generation limits for committed units. The planning problem will then be large scale and mixed integer and therefore cannot be solved by an exact method. Instead, a Lagrangian relaxation is used to decompose the problem into one subproblem for each hydro and thermal power plant. These subproblems are solved by dynamic programming. To find feasible solutions from relaxed solutions, a heuristic method based on network programming is used. The method is illustrated by a numerical example.