The invention discloses a simulation system estimating the self-heating characteristic of a circuit and a design method thereof. A method of designing a semiconductor circuit using a circuit simulation tool executed by a computer includes calculating power consumptions of elements of the semiconductor circuit by use of the circuit simulation tool. A thermal netlist is created about the semiconductor circuit, based on the power consumptions and geometry information of each of the elements. A simulation of the semiconductor circuit is performed with the thermal netlist using the circuit simulation tool to detect a temperature of each of the elements. The thermal netlist includes thermal capacitance information of each of the elements.