A non-primitive perturbation model for chain-like molecules has been used to correlate the activity coefficients of amino acids and peptides in aqueous solution. In this model, the mixed segments of hard spheres are taken as the reference and the chain formation, the dipole-dipole and L-J interactions are taken as the perturbation terms. The equation can accurately correlate the activity coefficients of amino acids and peptides in water with four parameters. The solubilities of amino acids in pure water between 0-100 degrees C can be predicted with the model parameters, and the experimental values of standard entropy and enthalpy changes in the dissolving process.