A series of critical clusters was prepared by mixing two different kinds of tetra-functional poly(ethylene glycol) (PEG) prepolymers carrying complementary end-groups. The structures of these critical clusters were investigated by small angle neutron scattering (SANS) in different dilution levels. Scaling laws for semidilute polymer solutions were observed in the solution of critical polymer clusters for q < xi(-1) where q is the scattering vector and xi is the correlation length. The fractal dimension of the critical clusters was estimated to be approximately 2.0, irrespective of the preparation condition of the critical dusters. For q > xi(-1) the size distribution of the critical clusters influenced the scattering intensity. Assuming the validity of the scattering theory for the dilute solution of critical polymer clusters in the q-range q > xi(-1) the Fisher exponent was estimated to be 1.90-2.2S, which was found to depend on the preparation condition of the critical clusters.