A projection apparatus includes a projection unit, an image processing unit, and a detection module. The projection unit receives an image output signal, and converts the image output signal into an image light beam for projecting a projection image. The detection module detects a placement orientation of the projection apparatus, and generates a placement signal according to the placement orientation detected thereby. The image processing unit is coupled to the projection unit and the detection module, and generates the image output signal to the projection unit. The image processing unit receives an image source signal, and adjusts the image source signal to result in the image output signal such that the projection image corresponding to the image output signal is rotated by a predetermined angle. The projection apparatus automatically detects the placement orientation and corrects the angle of the projection image, and user manual setting is no longer required.