Mechanism of the effect of hydrogen on propylene polymerization was investigated with supported TiCl₃/MgCl₂ catalysts having extremely low Ti contents. Hydrogen effects were found to depend on stereospecificity. Hydrogen had no effects on the catalyst activities of isospecific sites and aspesific sites. But, efficiency of hydrogen as chain transfer agent was different from isospecific sites to aspecific sites. Hydrogen was effective for chain transfer reaction on isospecific sites but was not on aspecific sites.