Four hundred and ninety-five Japanese primary-school children aged from 8 (Grade-2) to 12 (Grade-6) were tested for their abilities to read/write in Hiragana, Katakana, and Kanji, for their size of vocabulary and for other cognitive abilities including arithmetic, visuo-spatial and phonological processing. Percentages of the children whose reading/writing scores fell below the -1.5SD cut-off differ according to the scripts--Hiragana: 0.2% for reading and 1.6% for writing, Katakana: 1.4% and 3.8%, and Kanji: 6.9% and 6%, respectively. Further, for the normal children, the older the age, the better they performed on cognitive tasks, while the reading/writing disability (RWD) group (below -1.5SD) showed a weaker relationship between the age and the performance level. It was also revealed that for the normal children, the "vocabulary size" was the most potent predictor variable in accounting for Kanji word reading performance for all grades except Grade-6, for whom nonword reading/repetition were also significant predictor variables. In contrast, for Kanji word writing, generally other writing related variables were better predictor variables. The RWD group however showed different patterns of results. Thus the data from the normal and RWD children in the current cohort were discussed in terms of the current theories of reading, and developmental dyslexia.