A novel deterministic algorithm for solving medium term unit commitment（UC） problem based on outer approximation method（OAM） is presented. As a decomposition method， OAM decomposes UC problem into a sequence of mixed integer linear programming（MILP） master problems and nonlinear programming（NLP） sub-problems. Branch-and-cut method is used to solve the MILP and a new null space interior point method is used to solve the NLP. The simulation results tested on systems up to 54 units and 168 hours show that OAM can converge fast and handle the ramp rate constraints efficiently. Thus， OAM gives a new technique for solving large scale security-constrained unit commitment problems efficiently.