This paper presents an improved heuristic algorithm for solving combined heat and power economic dispatch (CHPED) problem in large scale power systems, by employing improved group search optimization (IGSO) method. The basic deficiency of the original GSO algorithm is the fact that, it gives a near optimal solution rather than an optimal one in a limited runtime period. In this paper, some modifications have been applied to the original GSO in order to improve its searching ability. In this work, a sinusoidal term is employed to consider the valve-point effects and Kron's formula is used in order to consider the transmission losses. Moreover, prohibited operating zones of power-only units are considered. The effectiveness and accuracy of the proposed method for solving the non-linear and non-convex problems is validated by carrying out simulation studies on sample benchmark test cases and three large scale power systems. Numerical results indicate that proposed method has the better performance in comparison with original GSO and some recently published papers. (C) 2014 Elsevier Ltd. All rights reserved.