<p>The letter-row tilt illusion is the illusion that the row is perceived to be tilted, when a set of letters is repeated in a physically horizontal or vertical row. We quantitatively examined the effects of aspect ratios of letters on the letter-row tilt illusion in horizontal letter-rows with or without a staircase structure of horizontal line segments. In Experiment <InternalRef RefID="Sec2">1</InternalRef>, the results quantitatively showed that the illusion significantly occurred in letter-rows with and without the staircase structure. In Experiment <InternalRef RefID="Sec9">2</InternalRef>, the results showed that the amount of illusion in letter-rows with the staircase structure increased as relative and absolute lengths of the horizontal line segments increased. In Experiment <InternalRef RefID="Sec12">3</InternalRef>, the results showed that the amount of illusion in letter-rows without the staircase structure had different tendencies in aspect ratios from that with the staircase structure. The present study suggested that different mechanisms were responsible for the letter-row tilt illusions with and without the staircase structure.</p>