Abstract: This study presents the comparative performance between two different photodiodes PIN and APD (Avalanche Photodiode). New development in this technology of both photodetectors opens a modern design for optical communication systems to improve the influence of several properties and parameters of those photodiodes which used for practical applications and different significant constructions. We use eye diagrams to evaluate the value of Q-factor in each case and every chosen cases to estimate the results. Therefore, the input power (dBm) and the fiber cable length (km) as well as bit rate (Gb/sec) have been used in this analyzed work to enhance the efficiency and performance for practical optical communication systems. These results implemented and analyzed by the OptiSystem Software through transceiver systems.