Application of ASF equation on the product distribution of FT synthesis using nanosized iron catalysts is studied. The dependencies of product distribution on the reaction temperature and reduction conditions of nanocatalysts are investigated by the growth probability of hydrocarbons (alpha). The latter has been calculated for three different FT catalysts under various operating conditions and hydrocarbon ranges (n). Since ASF equation may not be suitable for these systems, a new approach has been put for-ward. According to such an approach the molar product distribution is applied to calculate alpha values from the slopes of the plots of In (mol.%/n) against n. The latter are called the quasi-ASF plots. Quasi-ASF plots follow the same trends as those of ASF but with an improved linearity. (C) 2007 Elsevier B.V. All rights reserved.