Explanation and justification require cognitive ability which selects and organises relevant information in a logical way, and linguistic ability which enables speakers to encode the information with linguistic knowledge. This study aims to investigate the development of Chinese oral explanation and justification in Singapore primary students. The study comprised 36 participants ranging from six-year-old to 11-year-old. They were learning Chinese as a second language. A series of questions were designed to elicit explanations and justifications based on a story. Explanation and justification were analysed on length of discourse, frequency of connectives, types of explanation and types of justification. The results showed that, from a linguistic perspective, as age increased, the frequency of connectives and length of discourse increased first then decreased. This suggests that linguistic measures are not sufficient in revealing the development. From a cognitive perspective, the older participants produced more effective explanations of event-related causal relationships than the younger participants in regard to explanation. With regard to justification, the older participants integrated and analysed the information in the story to justify opinions, while the younger participants relied mainly on familiar information, such as moral principles. The results demonstrate that the development of Chinese oral explanation and justification in primary students is reflected on cognitive dimensions.