This invention relates to an three-constituent adduct of a metallocene.an ether.an metal halide having the general formula of Cp'MQ.sub.2.RXR'.nM'Q.sub.2/n, where Cp' is a substituted or an unsubstituted dicyclopentadienyl, M is zirconium or titanium, RXR' is tetrahydrofuran or diethylether, M' is alkali metal or alkali earth metal, n=1 or 2. The adduct can be exposed to air for several hours or stored in nitrogen atmosphere for a long period of time, which is prepared by means of a simple and convenient process without further separation and purification and used directly as a catalyst for polymerization of olefins.