Abstract: The information of targets scale is an important factor for small target detection in two-dimensional image. This paper presents a small target detection method based on second order directional derivative scale-space, chooses directly the interest target with the information of targets scale. On the basis of Lindebergs scale-space theory, the maximum and minimum second order directional derivative scale-spaces are derived, with the method of partial differential equations, and the characteristics of targets scale in the scale-spaces are analyzed. Meantime, the selection of parameters for constructing scale-spaces is given. On the analysis of the response of different targets in the scale-spaces, the implement algorithm using the second order directional derivative scale-space to detect small target is proposed. Experiments on the actual image show that the proposed method has a more robust detection performance than the compared one, and the detection capability of Laplace scale-space for non-circular structure target is improved.