The performance of a parallel flow micro channel heat exchanger (MCHX) is examined in this paper with the help of a mathematical model developed for this study. The thermal performance of a balanced parallel flow MCHX is numerically evaluated with the effect of heat transfer from the surroundings taken into consideration. Two non-dimensional governing equations are developed for modeling this MCHXPF. This model also considers a case in which the temperature of surrounding that is adjacent to the hot fluid channel is assumed to be different from that in contact with the cold fluid channel. The equations are numerically solved using the Runge-Kutta-Fehlberg method. MATLAB software is used to develop the program for solving the equations that constitute this model. The axial temperatures along both channels are obtained upon numerically solving the governing equations. The effectiveness of each fluid is then calculated using its inlet and outlet temperatures. The heat transfer between the fluids and that between the surrounding and each fluid are also numerically calculated using the axial temperatures of the fluids. The effectiveness of the fluids depends on the NTU, temperatures of the surroundings, and the thermal resistance between the individual fluids and their corresponding surrounding. When the heat transfer from the surrounding and the respective fluid is increased the effectiveness of the hot fluid decreased and that of the cold fluid increased. The temperature of the surroundings will determine the direction of heat transfer between the individual fluid and its surrounding. When the temperature of the surroundings is higher than the inlet temperature of the hot fluid then there would be degradation of the effectiveness of the hot fluid and an improvement in the effectiveness of the cold fluid. And the opposite trend happens when the temperature of the surroundings are lower than the inlet temperature of the cold fluid.