In terms of the low SNR and contrast, the small and dim targets can not be easily detected in a single IR image, so utilizing a sequence of images is an effective method. Correlativity of target in a sequence of images is analyzed for detection of small and dim target. Some advanced algorithms are discussed, and concept of contrast correlativity is set forth. The method abstains from the disadvantage due to direct using gray correlation in the case that the gray is fluctuant in the sequence of images, and eliminates the disturbance of noise and background. The algorithm could be applied in the small target detection in other types of image.