In the production of biodiesel the need for using more inexpensive and alternative raw materials arises. This is due to the limited amount of traditional raw materials in the market and their high prices. Very convenient alternative raw material which is also cost effective is used frying oil (UFO). The quality of the final product greatly depends on the quality of raw materials. By using only used frying oil (UFO) as raw material it is not always possible to obtain a good quality product. Therefore, it is necessary to make a mixture of raw materials to obtain a high quality product and at the same time to minimize the costs. The purpose of this paper is to investigate the impact of various raw material mixtures of rapeseed and used frying oil on the most important physico-chemical characteristics of biodiesel, viscosity and cold flow properties. It is shown that portion of UFO in raw material mixtures does not have significant impact on viscosity of final product, but only cold flow properties. We discovered linear correlations between cold flow properties of product formed from mixtures of raw materials, and cold flow properties of the product formed from pure raw materials. Cold flow properties of raw materials and cold flow properties of products are also in linear correlation. CFPP of the final product from the raw material mixture is equal to the sum of the product of portion of particular raw materials in the mixture and CFPP products from the same raw material. These findings can be implemented in determining optimal raw material mixture. We made a short overview of one such model, which is also included in this work. (c) 2012 Elsevier B.V. All rights reserved.