In-line digital particle holography is applied to measure the characteristics of spray droplets. Common reconstruction methods were considered and the best one was selected. Several important parameters at the time of hologram recording, such as the object distance and the region of laser beam used, are discussed. The feasibility of the correlation coefficient (CC) method for focal plane determination of 3-D droplets was verified. A double exposure hologram recording system with synchronization system for time control was established, and two digital spray holograms were obtained in a short time interval. For post-processing of reconstruction images, the two-threshold and the image segmentation methods were used in binary image transformation. Using the CC method and some image processing techniques applied to droplets in each double exposure image, the spatial positions of droplets used to evaluate the three dimensional droplet velocities were easily located, which proved the feasibility of in-line digital particle holographic technology as a good measurement tool for spray droplets.