Two model fines, representing microfibrils and microgranules, and two fractions of TMP fines were used to demonstrate their effect on tensile strength and optical properties of handsheets formed from TMP and kraft fibers. The model microfibrils behave as a binder between fibers, thus improving tensile strength. The light scattering of kraft handsheets decreases but that of TMP handsheets increases. The model microgranules behave as a pigment by improving light scattering but preventing interfiber bonding. TMP fines, containing both types are capable of increasing tensile strength and light scattering simultaneously. The relation between tensile strength and light scattering depends on the proportion of fibrillated fines and granulated fines. The fractionated TMP fines of high surface area are shown to be very effective in improving the handsheet properties and the relation between tensile strength and light scattering is superior to that achieved by calcium carbonate filler.