Abstract: Precipitation of n-alkanes in diesel affects cold flow property. With XRD pattern, shape of crystals is able to be studied. But XRD pattern has not been applied to investigation of precipitation of crystals of n-alkanes appearing in diesel. In this paper, XRD pattern of a series of diesel flow improver(DFI), wax and mixture of wax and DFI are analyzed. The relationship between character of XRD pattern and performance of DFI is studied. When sharp peak between 10°～30° is little, shape of peak is ordered and curve of peak is smooth, performance of DFI is better. XRD pattern of the mixture of wax and DFI shows noncrystalline structure has changed. Hence it might be considered crystallization procedure of wax be affected by DFI. This work is also helpful to the full understanding of the mechanism of DFI.