This research work emphasizes on the study of the computer assisted design and simulation of the differential CMOS operational amplifier. The main objective of the research was to allow op amps to maintain acceptable level of performance when the supply voltage is low. Because, low voltage and low power op amp design has become increasingly an interesting subject as many applications switch to portable battery powered operations. In this study, we have designed a two stage differential op-amp circuit and achieved an increased open-loop gain. The design has been implemented in 0.2µm CMOS process technology from a ±2.5V voltage supply and simulated using OrCAD PSPICE design and simulation software. We have achieved a unity gain band width of 4.0633MHz and an open loop DC gain of 134.245dB. The design was found to dissipate very small power (0.085mW). The phase difference was 139.533 0 . Key words: CMOS, two-stage differential input/ output op-amp, Stability, Slew Rate, Open loop DC gain, Power dissipation and OrCAD PSPICE.