The diagnostics of cracks in structures has been gaining importance in recent years. In the past decade, numerous sensors were developed to detect and monitor the crack propagation, but very few sensors can extract quantified information about the crack such as its size and orientation. Recently, we have presented a passive wireless antenna sensor that can detect sub-millimeter crack growth when the crack is parallel to one edge of the antenna patch. This paper studies the capability of the antenna sensor to detect crack orientation. Based on the principle of microstrip patch antenna, an antenna sensor with a rectangular patch radiates at two resonant frequencies. Cracks in the ground plane of the sensor with different orientations influence these two resonant frequencies in two different ways. Thus by monitoring the changes in both resonant frequencies of the antenna sensor, quantitative information about the crack orientations can be obtained. The resonant frequencies of the antenna sensor were first simulated using an EM simulation tool by modeling the crack as a 0.7 mm wide slot oriented at various angles. Simulation results confirmed that the resonant frequencies of the antenna sensor are sensitive to the crack orientation. Subsequently, the antenna sensor's capability to detect crack orientation was experimentally validated by fabricating an antenna sensor on a double-clad circuit board. A mini-milling machine was employed to produce cracks in the ground plane at different angles. The resonant frequencies of the antenna sensor were then measured at different crack lengths to study the effect of crack orientation and length on both frequencies of the antenna sensor. The principle of operation will be discussed first, followed by detailed descriptions on the simulation model, sensor design and fabrication, experimental setup and procedure, results and analysis.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.