In this research work, effect of dimethyl formamide (DMF) as an external electron donor in gas phase ethylene/1-butene copolymerization process in the presence of Ziegler-Natta catalyst has been investigated. Different experiments were performed using TiCl(4)/MgCl(2) as catalyst, triethyl aluminum as cocatalyst and in the presence of various amounts of DMF as an external electron donor. Then, effect of DMF on various parameters such as catalyst activity, molecular weight, and short chain branching content of the samples has been studied. The results showed that the more the ratio of DMF/Ti, the lower the catalyst activity. Moreover, the curves relating molecular weight and short chain branching content of the samples to the ratio of DMF/Ti passed through an extremum point at about DMF/Ti = 0.25. A justification for the occurrence of the extremum point has been proposed in this research work, and some analysis methods are adopted to confirm the suggested justification. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1267-1272, 2012