Experimental parameters were optimized by the analysis of variance (ANOVA) by D-optimal experimental design using three levels of pressure, temperature, weight hourly space velocity (WHSV) and catalyst particle size. The effects of the above parameters on the gasoline selectivity in methanol to gasoline conversion (MTG) were studied. Given the effect of catalyst particle size on MTG process, three HZSM-5 catalysts with particle sizes of 90, 2000 and 4000 nm were synthesized and the effect of this important parameter was studied along with those of other parameters. Fischer test results showed that catalyst particle size and temperature are the most and least effective parameters, respectively, in the ranges studied. The most important two-way interaction variable was that of temperature and WHSV. The optimal conditions to achieve maximum gasoline selectivity in maximum conversion are pressure = 9.40 bar, temperature = 392 degrees C, WHSV = 1 h(-1) and particle size = 994 nm. In addition, by implementing industrial conditions in the equation proposed by the experimental design, the optimal catalyst particle size was found to lie in the 2500-3300 nm range. (C) 2017 Elsevier B.V. All rights reserved.