To promote the biodiesel production, low-quality fats in addition to waste vegetable oils are still required. Low-quality waste vegetable oils and low-quality fats as feed mixtures contain free saturated fatty acids and/or commercial solidifiers in addition to triolein as a main component, and they are solid at room temperature. However, liquid feed is desired for biodiesel production processes. In this study, solidification properties of a system of commercial solidifier and commercial vegetable oil as low-quality fats and a binary system of palmitic acid and triolein as low-quality vegetable oils were investigated, and the melting temperature was calculated by a simple thermodynamics function of composition. The binary mixture of palmitic acid and triolein could be liquefied at above 65 degrees C regardless of the composition, and the melting temperature of the mixture could be represented by the simple thermodynamic function. On the other hand, the melting temperature of the mixture of the commercial solidifier and commercial vegetable oil increased sharply to 80 degrees C even at low solidifier concentration, and the melting temperature of waste fats could not be represented by the simple thermodynamic function.