A novel optical double-image encryption scheme based on Gaussian apertured reality-preserving fractional Mellin transform (GARPFrMT) is proposed. In terms of optical image encryption, GARPFrMT has several attractive advantages: (1) The GAPRFrMT can be used to scramble the value of two images simultaneously; (2) GARPFrMT is constructed in diffraction domain to ensure that the ciphertext is real; (3) Owing to the nonlinearity, this transform can ensure that the proposed encryption system can be kept from the potential insecurity; and (4) A Gaussian lens is applied to improve intensity distributions of the output light. Experimental results demonstrate that the proposed algorithm is feasible and robust by a lot of attack analysis, i.e. sensitivity attack analysis (>99.60%), differential attack analysis (NPCR >99.6072 and UACI >33.5681%), entropy values (almost 7.99928), statistical attack (correlation coefficient values are almost 0), noise attack, cropping attack and NIST analysis.