Effects of tener size ( d ₅₀) and toner surface-charge density ( q/s_t ) on electrophotographic imagequality factors (i.e. image density for solid area, resolution of line images and reproduction of dot images) have been analyzed by varying the toner surface-area median size ( d ₅₀) from 6.8 to 13.5 μm and the carrier surface-area median size ( D ₅₀) from 43 to 106 μm in two-component development systems employing the toner and the insulative carrier. As regards the optical density of toner image for solid area, development efficiency is determined by the toner surface-charge density ( q/s_t ), independently of the toner size ( d ₅₀) and the carrier size ( D ₅₀), not by the toner charge-to-mass ratio ( q/m ) as suggested by Schein and by others. As regards the resolution of line images produced in the rotating direction of the photoreceptor drum, the width of a line broadens by four particles of the toner, ΔΓ =4· d ₅₀. The resultant resolution for lines is determined by the reciprocal of the broadening width ( ΔΓ ).