We address the problem of detecting targets with different areas in air background, which will be very complex when containing part of ground. A target detection approach based on morphological gradient is proposed. Firstly, morphological gradient of the IR frame is computed. Then the air region is gotten based on the morphological gradient. At last, targets in the air region are detected and labeled. Because morphological gradient is sensitive to the slow changing grayscale on IR targets’ boundaries, morphological gradient is used to strengthen targets’ boundaries and detect targets by detecting the boundaries. Our experiments show that the method yields good detecting performance and is suitable for detecting targets with different areas.