A new kind of feature of an image was presented on the basis of the self similarity property of the fractal model, which could be applied to recognize man made objecs in the natural background. Firstly, the interpretation of the feature was discussed and the procedure of the computation of the feature was presented. Secondly, the feature was compared with the fractal dimension feature and the fractal model fitting error feature based on the experimental data to show its superiority in recognizing man made objects in the natural background. Finally, some experimental results for practical infrared images were presented to show the effect of the feature in the practical situations.