This paper presents a solution technique for combined emission constrained unit commitment problem (UCP) . The emission level is taken as a constraint in both the selection of units for generation and also in dispatching the real power among the committed units. The multi objective problem is converted into a single objective using max price penalty factor method. Since unit commitment problem is highly constrained and complex, we need a solution technique capable of solving such complex problems. Improved Shuffled Frog Leaping Algorithm (SFLA) is a memetic algorithm which deals with the behaviour of group of frogs searching for the location that has the maximum amount of available food. Leaping of the frog is improved by the introduction of cognitive component. This ensures the faster convergence and global optimal solution. The integer coded UC is used which avoids any extra penalty function for satisfying the minimum up/down constraint. The SFLA is used in two stages in this proposed method. This proposed algorithm has been implemented in Matlab 2011 environment. IEEE 14 bus system, IEEE 30 bus system, IEEE 56 bus system and IEEE 118 bus system are taken as the test system. We have taken 3 cases such as purely economical case, purely emission case and combined emission and economic case by varying the weighting factors for the constraints. The results of these cases are discussed to explain the effect of emission in selecting units and in economic dispatch.