A process for producing a modified particle, which comprises the step of contacting a compound (a) defined by the formula, M.sup.1L.sup.1.sub.3, a compound (b) defined by the formula, R.sup.1.sub.t-1TH, a compound (c) or (e) defined by the formula, R.sup.2.sub.m-uM.sup.2(OH).sub.u or R.sup.2.sub.4-nJ(OH).sub.n, respectively, and a particle (d) with one another; a carrier comprising a modified particle produced by said process; a catalyst component (A) comprising a modified particle produced by said process; a process for producing a catalyst for addition polymerization, which comprises the step of contacting said catalyst component (A), a transition metal compound (B) and an optional organoaluminum compound (C) with one another; and a process for producing an addition polymer, which comprises the step of addition polymerizing an addition-polymerizable monomer in the presence of a catalyst for addition polymerization produced by said process.