The power-law dependences of critical amplitudes either on the molar volume of cycloalkane or on the interaction parameter per unit volume for coexistence curve, correlation length and susceptibility have been derived from a Landau-Ginsburg-Wilson model and the Scatchard-Hildebrand theory for solutions of cycloalkanes in a polar solvent. Coexistence curves of (T, n), (T, x) and (T, φ), where n, x, andφare refractive index, mole fraction and volume fraction respectively, for the binary mixtures of methylcyclopentane and cycloheptane in N-methyl-2-pyrrolidone (NMP) have been determined in the critical regions by measurements of n. The critical amplitude B and the critical exponent b have been deduced. The experimental results reported in this work and previous work are in good agreement with theoretical predictions.