Economic dispatch is an important issue in electrical power system planning and operation. Its aim is to dispatch electricity to the consumers at minimal economic cost, while satisfying system constraints. Prohibited operating zones is one of the constraints which requires attention. This paper presents an efficient optimization technique termed as Immune Log-Normal Evolutionary Programming (ILNEP) to solve economic dispatch problem with prohibited operating zones. The proposed ILNEP optimization technique to solve economic dispatch problem with prohibited operating zones has been tested on the IEEE 26-Bus Reliability Test System (RTS). The proposed method has been tested on three case studies with different load conditions, for validation of its effectiveness. The three case studies are lightly loaded, mediumly loaded and heavily loaded. Comparison of results among ILNEP, artificial immune system (AIS) and evolutionary programming (EP) have been made and it revealed that ILNEP outperformed AIS and EP in giving the lowest total production cost.