This paper presents mathematical modelling and simulation of the thermal behavior in VLSI (Very Large Scale Integration) circuits. The mathematical model is based on the formulation of Heat Transfer equation and is discretized by finite difference method. The algorithm developed in Matlab® implements a computational model of a thermal transfer on substrate of VLSI circuit with arbitrary function and dimensions. Results are presented in a series of 3D images for different times of VLSI circuit. 3D images show temperature in z axis, dimensions in (x, y) axis and 2D images show temperature in color scale, dimensions in (x, y) axis. Last, this work present a relation between real time of simulation and the equivalent time of circuit's work.