This study investigated the feasibility of preparing biodegradable composites, such as food packing materials, from plant leaves as a substitute plastic. Lotus leaves, reed leaves, and basho leaves were treated with hot water and combined with gelatin to form composite samples. The effects on the morphology, thermal properties of leaves, and the mechanical properties and moisture absorption of the composites were studied. The Fourier-transform infrared spectroscopy (FTIR) analysis showed that the amorphous components such as lignin, wax, and pectin were removed after the hot water treatment. The treatment had the most beneficial effect on the reed leaf. The reed/gelatin composites had the best mechanical properties, of which the tensile strength and the flexural strength were 14.0% and 77.1%, respectively, higher than that of the lotus/gelatin composites and 121.5% and 192.5%, respectively, higher than that of the basho/gelatin composites. The morphology of the cross-section of the composites showed that there were numerous holes and gaps in the basho/gelatin composites which induced a high moisture absorption performance.