According to the present invention, there are provided a catalyst and a process for the preparation of polyolefins having a relatively wide molecular weight distribution, good particle properties, a narrow composition distribution in copolymerization and a high molecular weight and being less sticky. The catalyst is prepared by contacting the following constituents (1) to (4) with one another:(1) a compound represented by the general formula Me.sup.1 R.sup.1.sub.p (OR.sup.2).sub.q X.sub.4-p-q, where Me.sup.1 is Zr, Ti or Hf, R.sup.1 and R.sup.2 are each independently a hydrocarbon group having 1 to 24 carbon atoms, X is a halogen atom, p and q are each an integer in the ranges of 0.ltoreq.p.ltoreq.4 and 0.ltoreq.q.ltoreq.4 , provided 0.ltoreq.p+q.ltoreq.4;(2) a compound represented by the general formula Me.sup.2 R.sup.3.sub.m (OR.sup.4).sub.n X.sup.2.sub.z-m-n, where Me.sup.2 is a Group I-III element in the Periodic Table, R.sup.3 and R.sup.4 are each independently a hydrocarbon group having 1 to 24 carbon atoms, X.sup.2 is a halogen atom or a hydrogen atom, provided when X.sup.2 is a hydrogen atom, Me.sup.2 is a Group III element in the Periodic Table, z is the valence of Me.sup.2, m and n are each an integer in the ranges of 0.ltoreq.m.ltoreq.z, and 0.ltoreq.n.ltoreq.z, provided 0.ltoreq.m+n.ltoreq.z;(3) an organocyclic compound having conjugated double bonds; and(4) a modified organoaluminum compound containing Al--O--Al bond, and thereafter contacting the resulting contact reaction product with the following component (5):(5) an inorganic carrier and/or a particulate polymer carrier.