A new method for winterization of biodiesel based on waste cooking oil is demonstrated, using micro heat exchangers with channel diameters of 200 pm. Biodiesel is pumped from a vessel through a micro heat exchanger in such a way, that pure seed crystals of saturated fatty acid methyl esters are produced at the outlet of the micro channels and injected back into the biodiesel vessel. Thus micro process engineering allows the reduction of the sum of saturated fatty acid methyl esters within biodiesel based on waste cooking oil from 21.3% to 9.6%. This corresponds to a reduction in CFPP value of 11 K, which means that this biodiesel can be used at temperatures down to 264 K. (c) 2008 Elsevier Ltd. All rights reserved.