In this paper, an algorithm to solve environmental constrained unit commitment problem (UCP) with operational and power flow constraints has been developed to plan an economic and secure generation schedule. Both Economic load dispatch (ELD) and Economic emission dispatch (EED) have been applied to obtain optimal fuel cost and optimal emission of generating units for the entire time horizon. The unit commitment solution for the environmental constrained problem has been formulated as a multiobjective problem by considering both ELD and EED simultaneously. The common economic emission dispatch (CEED) bi-objective problem is converted to single objective function by adding a price penalty factor. A modified price penalty factor is proposed to solve this problem. The UCP solutions without operational and power flow constraints are not practical due to secure operation of the power system network. This proposed algorithm introduces an efficient unit commitment (UC) approach considering environmental constraints along with power flow constraints that obtains the minimum operating cost satisfying both unit and network constraints. In the proposed model repeated optimal power flow (OPF) for the satisfactory unit combinations under the given study period has been carried out to obtain UC solutions with both operational and power flow constraints. This proposed algorithm has been tested for environmental constrained UCP on IEEE 30 bus and Indian utility practical systems with and without power flow constraints scheduled for 24 hours. The solutions obtained are quite encouraging and useful in the economic emission environment. The algorithm and simulation are carried through Matlab environment. © 2006 IEEE.