Effect of H-2 amount on the activity and selectivity of Chevron-Phillips trimerization system, i.e., chromium(III) tris(2-ethylhexanoate)/2,5-dimethylpyrrole/triethylaluminum (TEA)/tetrachloroethane (TCE), was examined. In this regard, in addition to catalytic ingredients and ethylene monomer, 2.5 and 5 bar H-2 were utilized in the oligomerization reactor and the reaction results were compared with those without H-2. Results showed that addition of hydrogen has a remarkable effect on polymer formation and catalyst activity so that 147% decrease in polymer formation and 38% increase in activity were observed upon addition of 5 bar H-2. However, its effect on 1-hexene selectivity was not pronounced. Then, molecular modeling with DFT method was employed to simulate oligomerization process in molecular level. Energy results revealed that formation of 1-hexene has less energy barrier than the formation of 1-butene and 1-octene which confirms higher selectivity of Chevron-Phillips system toward 1-hexene formation. Furthermore, interaction of hydrogen with catalytic active center was studied via hydrogenolysis of metalacycles. Energy results revealed that it is energetically a facile process so that the bigger the circle, the easier hydrogenolysis. [GRAPHICS] .