Latent image formation on electrophotographic Laser beam printer (LBP) which uses Gaussian laser beam and biased voltage development was investigated. As surface voltage of photoreceptor induced by laser exposure must be under the biased voltage, threshold laser power was determined on a simple form. The calculation was compared with experiments of single dot development by 8μm toner with 800dpi and 60ppm LBP. Although toner image of the single dot printing was blurred by the process of development, quantitative agreement was obtained.