This study proposes a model of adaptive luminance difference between text and background for comfortable reading on a smartphone display. The study is composed of two experiments. In Experiment I, the optimal luminance difference is identified in accordance with reading speed and preference. On the basis of the experimental results, the gradual decrease of luminance difference between text and background is developed. The change occurs while reading the text, and the model is applied to various illuminance levels. In Experiment II, the effect of adaptive luminance difference is validated in terms of reading speed, preference, and brainwave analysis using an electroencephalogram. Empirical evidence confirms that the developed model improves physiological comfort and psychological satisfaction, thereby it has a potential to be applied to the visual display industry.