In this Letter, we present a theoretical analysis of isotope effect on the structure of He-3-He-4 fluid mixture based on a generalized Omstein-Zernike (GOZ) integral equation for quantum solutions. The theory is an extension of our GOZ for pure quantum fluids to the quantum solutions. Thermodynamic quantities such as potential and kinetic energies and partial molar volume as well as microscopic fluid structure were analyzed for a He-3 atom dissolved in the He-4 solvent, i.e.. the solution at infinite dilution at T = 40 and 4 K. Our integral equation theory could successfully describe the isotope effect on the quantum solution.