The second virial coefficients Bij (nm3) of poly(ethylene glycol) with molecular weight M = 600-104 in water are measured by freezing point depression. B12 represents the virial cross coefficient for two PEG solutes with different molecular weights M1 and M2. B11 can be well described by the scaling law M3v with v ≃ 0.60. That is, the good solvent behavior is observed even for such low molecular weight. In terms of the hard-sphere model, the effective diameter of PEG ranges from 1.3 to 7.9 nm. Since the second virial coefficient is generally increased with decreasing temperature, our results at freezing point provide an upper bound. The virial cross coefficient is found to follow B12 = B11[1 + (M2/M1)v]3/8. This result indicates that the effective hard-sphere picture is also reasonable for dilute solutions of different polymer molecules in good solvents.