Abstract: The invention is to provide an improved process for preparing polyolefins having predetermined properties, in particular, melt index and/or density by polymerizing olefin in the presence of a Ziegler catalyst and hydrogen. The parameters corresponding to the concentrations of olefin and hydrogen, participating in the properties, in the gas phase of reactor are detected by high-speed gaschromatography. The detected signals are applied to a computer.The feed volumes of olefin and hydrogen into the reactor are controlled by means of an operation control output, and the respective concentrations in the reactor are controlled. Consequently, the resulting polyolefins possess extremely stable and uniform properties.