Abstract: This research comes to determine the optimum tilt angle for PV system to be considered when fixing the PV systems. The optimum tilt angle is normally close to the latitude value of the location, lots of other factors affect this. So, when designing a PV system, it is necessary to take these factors into consideration to maximize the efficiency of the PV systems. The present research investigate the main scientific parameters related to solar radiation, the models used to describe the solar radiation properties and the methods to calculate the optimum tilt angle. The optimum tilt angle for Irbid, Ajlun Amman and Zarqa is calculated by applying the appropriate methods and models. Other factors affecting the tilt angle of a PV array are reported and relevant recommendations are given.