Orthotropic steel decks (OSDs) have been widely used in long and medium-span steel bridges because of superior structural features such as lightweight, high load carrying capacity, and modular construction. However, OSD is reported as one of the most structures experiencing fatigue defects which could limit the service life of the bridge deck. Recently, Ultra-High-Performance Concrete (UHPC) has been employed as a top layer in OSDs to improve the whole stiffness of OSDs and enhance their fatigue resistance. In the current study, numerical investigation has been performed to explore the stress response at fatigue-prone details of OSDs under the effect of repeated vehicle loading. In addition, the effect of employing UHPC layer on the hot spot stress at sensitive fatigue locations has been investigated. Quadratic extrapolation approach has been adopted to estimate the hot spot stress at the weld toe of the considered welded details. Results indicated that the stresses at rib-to-floorbeam welded connection are the highest stresses among the considered fatigue-prone locations and thus more concerns should be paid for the enhancement of its fatigue resistance. The application of a 50 mm thick UHPC layer considerably minimized the hot spot stresses at critical locations of all the considered fatigue-prone details in OSDs which indicates the efficiency of employing UHPC layer in OSDs for improving their fatigue strength and providing an infinite fatigue life.