An experimental study was conducted to examine how people feel and judge the apparent length and slimness of legs wearing sportswear with stripy designs, and to identify what type of stripy designs for sportswear provides visual effects making legs appear longer and slimmer. Consequently, the sample without stripes on a white background was rated short and thick while all the samples with stripes on white backgrounds were rated rather long and slim, proving that a geometrical illusion effect can be obtained by arranging black stripes on a white background. Furthermore, the test results showed that increasing the thickness and number of stripes on either a white background or a black background would make legs appear to be longer and that increasing the thickness of stripes on a white background would make legs appear slimmer, and a black background itself even without stripes would have an effect of compacting the appearance and making the legs appear rather slim. Among the white-background samples, “2 stripe of 6mm thickness”, “3 stripe of 10mm thickness”and “1 stripe of 25mm thickness”were rated for making legs appear long and slim. Among the black-background samples, the“3 stripe of 10mm thickness”sample was rated for making legs appear long and slim.