Abstract: A process for preparing 4-methyl-1-pentene comprising dimerizing propylene in the presence of a catalyst, characterized in that the catalyst is composed from the treatment of precatalyst by at least one oxygen-containing compound selected from oxygen, alcohols, ethers, acetals, orthocarboxylates and esters. The precatalyst is composed of element or compound selected from sodium, potassium, sodium amide and potassium amide supported on a carrier principally represented by the following formula (1):K.sub.2 O.xAl.sub.2 O.sub.3 (1)wherein x has a value in the range of 0.5.ltoreq.x.ltoreq.11. In one embodiment, the precatalyst is hydogenated prior to the use thereof.The use of above mentioned catalyst so remarkably increases the rate of reaction and the selectivity of 4-methyl-1-pentene and allows the reaction rate and selectivity to be kept high for a long period of time.