Radio Frequency Identification (RFID) system uses the principle of radiative power transfer between the reader and the tag antenna. The main performance metric for RFID system is the reliable reading coverage, where the tag can be read with higher detection probability. Most of current researches consider the reader coverage to be determined only by its read range assuming monostatic configuration with omni-directional antennas. In this paper, we model and study the effect of cascaded channel fading and readers antenna orientation on the passive RFID tags, in terms of detection probability. We derive a closed-form expression for passive RFID detection probability taking into consideration the relative reader-tag antennas orientations and cascaded Rician-forward/Rayleigh-reverse fading channel. The derived formulas can be useful for design and optimization of passive RFID communication systems from RF point of view.