In homogeneous chromium-based catalyst system [Cr(III) tris(2-ethylhexanoate)/2,5-dimethylpyrrole/AlEt3/halide], the effect of halide compound to 1-hexene formation selectivity and catalytic activity was investigated. The comparative studies showed that the promotion effect of a chloride was much better than that of a corresponding fluoride or bromide and the best kind of halides is multi-chloro compound which shows significant promotion effect to assist chromium center to achieve high 1-hexene formation selectivity and highly catalytic activity. In catalyst system, facial coordination geometry of chromium center is much more favorable for ethylene trimerization than meridional coordination geometry. The effect of halide could be interpreted well from the stabilization on facial geometry of chromium complex through the coordination of halo group with chromium center in some specific ways. Under the guide of the roles proposed on halide component, a new catalyst system [Cr(2-ethylhexanoate)(3)/2,5-dimethylpyrrole/AlEt3/2-fluoro-6-chloro-alp ha, alpha, alpha-trichlorotoluene] was developed successfully, demonstrating both high 1-hexene formation selectivity and highly catalytic activity. (C) 2004 Published by Elsevier B.V.