We investigated the effect of character spacing on reading speed in Korean sentences. The spacing conditions were manipulated as 0.5, 0.75, 1, 1.5, and 2 times the standard spacing on the internet. Spacing effects were examined in three different character sizes - 0.5, 1, and 2 times the standard character size on the Korean Internet. The reading times for sentences were converted into reading speed (words per minute).\nCritical spacing and maximum reading speed were also estimated. The reading speed increased with spacing, up to a critical spacing, and then remained constant and decreased slightly for the larger spacing.\nThe changing pattern of reading speed as a function of spacing was magnified for the largest character size. The critical spacing was 75% of the standard spacing. The results confirmed that spacing between Hangul characters is a critical variable which affects reading speed systematically in Hangul sentences.\nFurther, character spacing was interacted with character size. It is necessary to use an appropriate level of spacing for effective reading. Specifically it is desirable to reduce spacing from the standard spacing on the Korean Internet environment.