This research examined the acquisition of human/animal numerical classifier categories in preschool children. In Experiment 1, using an error detection paradigm, we examined the developmental change of the knowledge 150 children possessed about classifiers. Overall, even the youngest group of children (early 4-year-olds) were aware of the grammatical function of classifiers, but they did not know the semantic criteria for each classifier category until around their late 5 years of age. Experiment 2 examined 60 4-and 5-year-olds who had not shown the sensitivity for the semantic rule dividing the hiki (small animal) category and the tou (large animal) category. The results showed that the 5-year-olds only, could spontaneously generate the correct rule and apply it to new items only through hearing adults' use of classifiers. Based on such results, the interplay between the linguistic input, grammar and extra-linguistic cognitive categories in acquisition of classifiers was discussed.