This paper deals with modeling and optimization of under-floor heating by Pex pipes. For modeling, the calculation region is divided into two sections. First section is above the floor comprising room space and its walls. For this section, the Multizone model is used. In this model, the first section is considered as a Control Volume and its non-linear energy equations are solved by Newton- Raffson method. Second section is the floor and below comprising heating pipes, concrete and floor covering. This section is modeled by numerical method. Through combining the modeling of first and second sections simulation of the total system is completed. For optimization of under floor heating system, a goal function has been defined. Pipe diameter, concrete thickness, type and thickness of floor covering, and hot water flow rate inside the pipes are considered as deciding variables (design parameters). In continuation, sensitivity of the goal function to variation of deciding variables is investigated. Finally, effects of atmospheric temperature changes and thermal insulation thickness of the building upon the design parameters are investigated.