The purpose of the present study was to clarify the effects of the complexity of the Chinese letter on learning to read it. It was hypothesized that the Chinese letter is basically composed by attaching the meanings to special physical pattern. Characteristics of the pattern was defined by the physical complexity. And the complexity (C) was measured by simply counting the numbers of the parts (isolated lines, segments and dots) of the letter, regardless of it's meanings. First, in order to get some indication of validity of the measure, the relationships between the C and use frequency of the Chinese letter were investigated. We got, as we expected, a linear relationship between the C and the logarithm of the order of the frequency, and the lower the frequency of the usage, the greater the C of the letter. Second, paired-associate learning between the nonsense physical pattern composed randomly with lines and meaningful word was conducted. The stimuli were 12 physical patterns that were divided into six different C levels, and two irregularity (IR) levels. The responses were 12 two-syllabic meaningful words. The results were: (1) Learning trials were statistically significant for the level of the C, but not for the level of the IR.(2) The less complex patterns (level I) were most easy to learn.(3) Except at level I, there was a tendency for the more complex (MC) patterns to be easier to learn. Third, the effects of the C and use frequency (UF) of the Chinese letters on reading them were tested with normal adults and college students, defining the less complex (LC) letters to be the patterns that are equal to or less complex than 15C, and the MC letters to be the ones that are equal to or more than 30C. Then 160 letters were selected from each group. The results were: (1) The errors of reading were significant for the level of the UF, and the higher the UF of the letters, the easier to read them.(2) The MC letters were significantly more readable than LC letters when the UF were constant. Those results were discussed by making a distinction between the stimulus differentiation phase and the S-R association phase in the learning of letters. It was suggested that the stimulus differentiation phase is established easier and much faster than S-R association. MC letters have more information or cues that promote the S-R association than do LC ones.