The thermal behaviour of semiconductor devices has attracted increasing attention in recently. The phenomenon of heat generation and dissipation has been a challenge ever since semiconductor technology has evolved -. High power generating devices like power diodes, transistors, thyristors, etc. raise their temperature in excess of acceptable limits for normal operation . Device parameters like threshold voltage, mobility, carrier concentration and band gap reference, being temperature sensitive, change and in turn change the performance of the designed circuit Concern till date on thermal modeling of integrated circuits has been primarily confined to the design of electro-thermal bipolar integrated circuits -. In MOS domain all work has been done in the modeling of behavior of discrete MOS devices at high temperature. In MOS Circuits, work has been done in the modeling of the behavior of discrete MOS devices at high temperature. For complex systems where exact analytical models cannot be obtained, numerical models can be used with some approximations to simulate the behavior of actual systems . The scope of this present work has been analysis of the thermal behavior of a MOS integrated circuit, and development of a mathematical model to estimate the integrated circuit output parameters when the various devices in the circuit get heated to temperatures in excess of their normal operating range. This analysis becomes even more necessary at such high levels of integration as in VLSI where strong thermal interactions are unavoidable. Thermal effects are manifested in two ways . Firstly, uniform heating of integrated circuit chip  leads to a uniform change in temperature sensitive parameters of all devices on the semiconductor chip. Secondly, localized heating at certain areas on this chip causes some devices inside it to dissipate more heat and have higher temperature than others. This unequal heating of different regions introduces a thermal gradient on the chip due to which heat flows from high heat dissipating hot regions to the relatively colder ones. Consequently, the temperature sensitive parameters of all devices on the chip do not change uniformly. Hence, we propose a numerical model to estimate the behavior of an integrated circuit due to unequal heating of different elements within it. The circuit chosen for analysis is the MOS differential amplifier.