Worldwide Interoperability for Microwave Access (WIMAX) is a technology based on IEEE 802.16 standard. As one of its main features and applications, WIMAX is to be used as the solution for the last mile access for both fixed and mobility applications . IEEE 802.16 standard was developed to deliver Non Line of Site (NLOS) connectivity between a Subscriber Station (SS) and Base Station (BS) with typical cell radius of three to ten kilometers. This thesis is a study of WIMAX performance using Optimized Network Engineering Tools 14.5 (OPNET14.5) Internet Protocol version 4 (IPv4) model for fixed and mobile WIMAX. Several features were studied such as Automatic Repeat on reQuest (ARQ) for controlling errors techniques, Quality of Service (QoS) for different applications, pathloss propagation model and WIMAX handover. The simulation results of using ARQ shows high retransmission efficiency, the goal is to guarantee the reliable delivery of data packets. The ARQ provides the ability to significantly improve the throughput from approximately 50% in the case of 50% packet drop and no ARQ, also achieve a throughput of about 75% when increasing the ARQ in one node and reaches 100% when increasing the ARQ in two nodes. QoS parameters for Voice over Internet Protocol (VoIP) and File Transfer Protocol (FTP) applications were studied. Five different service classes were examined and discussed to study the effect of each class to those applications (Unsolicited Grant Services (UGS), real time Poll Services (rtPS), non real time Poll Services (nrtPS), extended real time Poll Services (ertPS) and Best Effort (BE)). ertPS scheduling class which has the advantage of giving back some of its reserved bandwidth, if there is no traffic to be served by this bandwidth. For voice quality over different scenarios, the performance of both voice and data show how during ertPS idle periods, BE increase its transmission rate up from 100 Kbps to ~200 Kbps. This is because ertPS deallocates its resources (to a minimum) during idle periods. Pathloss parameters were studied in various environments such as free space, outdoor to indoor and pedestrian, urban and suburban to show the effect of the distance between the transmitter and the receiver in different operating frequencies on the BS coverage area. The pathloss in each environment is increased when the distance between the transmitter and the receiver increased. The simulation results of WIMAX mobility and scanning shows that increasing scan duration from 4 to 20 frames may reduce the average application throughput from 150 Kbps to 140 Kbps and increase average delay from 0.89 sec to 2.5 sec. This should be expected since the WIMAX will be blocked more often while it is scanning neighbor BSs. The simulation results of ranging connectivity between the mobile station and the base station show that when the range of power tolerance at each BS is increased, the sensitivity also increased so the coverage area of each BS will increase. The results were satisfactory for all scenarios under different conditions and the analysis on the WIMAX performance were acceptable.