A novel method of digital image encryption is presented. Image encryption and decryption are performed based on the continuously increasing decorrelation property and the real-valuedness of the reality-preserving fractional Fourier transform. The input and encrypted data are respectively in the spatial domain and the reality-preserving fractional Fourier transformed domain determined by encryption keys. It resists statistic analyses effectively. In addition, the encrypted image is real-valued, so it?s convenient for display and storage. The simulation result shows that the parameters of the reality-preserving fractional Fourier transform enhance the space of keys. As a method of encryption, it won?t bring data expanding, and is sensitive to parameters, with considerable robustness, security, and feasible implementation in practice, which has a good prospect and practicability in information security field.