Abstract: A process for preparing cis-polyisoprene which comprises polymerization of isoprene in a medium of a non-polar hydrocarbon solvent at a temperature ranging from 0.degree. to 60.degree. C in the presence of a catalyst comprising a reaction product of titanium tetrachloride and an intracyclic organo-aluminium compound having the formula: ##STR1## wherein R, the same or different, are H, CH.sub.3 ; taken in a molar ratio of 1:1.05-1.1 respectively; weight ratio of isoprene to said catalyst being equal to 100:0.2-0.7.The present invention makes it possible to improve the quality of the thus-obtained cis-polyisoprene and to increase the catalyst capacity by two times due to modification of its properties.