Utilization of hydro pumped-storage (PS) units in conjunction with thermal generating units (TGU) could serve not only as an effective means for providing ancillary services and levelizing demand curve but also as a mechanism for lowering costs for TGUs operation. The objective of this study is to develop a multi-constraint (leveling demand curve, providing spinning reserve, and decreasing fuel and start-up costs (TC)) solution of the unit commitment (UC) problem with TGUs and considerations for PS units (PSUC). After determining the output power of PS units, a new optimization methodology (NOM) is examined to solve the UC problem for TGUs. The NOM benefits from a newly developed fitness function. The application of proposed methodologies is examined for power systems, used in other studies, for a 24 hour operation period. For the UC, the results show improvements between 0.01 and 4.16% in TC, as compared with the best results reported in the literature. The PSUC results show that the savings in TC for this study, when PS units are dispatched for leveling demand curve and providing spinning reserve, are 2.62 and 2.65%, respectively. Also, as a result of utilizing PS units, the demand curve is leveled and all of spinning reserve requirement is met. Copyright© 2010