According to the characteristics of the infrared images,an image enhancement algorithm based on sine gray level transformation is proposed.The whole image is divided into three areas according to the size of gray level with two inflection points and the three areas are background area,target area and transition area.Different gray level mapping transformation is applied to different areas.A sine function is adopted for the gray level stretching whose derivative is smooth and its gray level scalability is good.The inflection points of stretching and the parameters of stretching intensity are independently adjustable.The algorithm is also versatile and simple.The experimental results verify the effectiveness of the algorithm.