The purpose of this study was to find the clue clarifying the acquistion process of the JOSHI (particles). This was done by analyzing the difficulty in the JOSHI GA and O acquired by comparing the kind of sentences using JOSHI. The subjects were 80 normal children with a vocabulary age between 4:0 and 7:11. In order to test the subjects' ability to use JOSHI, 33 incomplete sentences without JOSHI were presented to them using a taperecorder and picture cards. Then, they were asked to repeat while completing the sentence after listening to each incomplete sentence. The results of this experiment might be summarized as follows: 1. The children with a vocabulary age between 7:0 and 7:11could use “GA” and “O” almost perfectly. 2. The ability to use “GA” was acquired earlier than the ability to use “O”. 3. The child not knowing the accurate meaning of “GA” and “O” had a trend to use “GA”. 4. The dificulty to use “GA” depended on a) whether the sentence described the situation that could be described by other similar sentences or not. b) what the kind of noun used as the subject in the sentence. c) whether the intransitive verb requiring the use of “GA” had a corresponding transitive verb or not. 5. The difficulty to use “O” depending on whether the sentence would be a reversal or nonreversal sentence.