This dissertation examines the heritage language (HL) ability of school-aged Mandarin HL children (age range = 6;5 – 10;10) living in Edmonton, a Canadian English majority city, and the factors contributing to their HL development and maintenance. Children’s language data were collected via various tasks, including comprehension, production, grammaticality judgment, psycholinguistic experiments, and narratives. A parental questionnaire was also included to collect Mandarin HL children’s information about the onset of English exposure, home language use, the richness of language environment, and the maternal education level. The results from three independent studies showed that Mandarin HL children were different from their monolingual peers regarding acquiring the lexically-driven classifier system and aspect morphemes. However, they were comparable to monolingual norms in comprehending and producing various syntactic structures (e.g. post-verbal clauses, relative clauses). These results indicate that the phenomenon of incomplete acquisition does not occur in every linguistic subdomain. Moreover, the longitudinal results revealed that Mandarin HL children’s L1 was convergent with the target grammar over time, suggesting that the reduced exposure to the HL does not necessarily lead to incomplete acquisition and attrition in the HL. Regarding various language environmental factors, the results demonstrated that older age of arrival, a rich and diverse HL environment, higher maternal education level and bilingual education contribute to stronger HL abilities, pointing to an important role for input in HL acquisition. Taken together, this dissertation offers insights relating to the debates between incomplete acquisition, attrition and the protracted acquisition in the acquisition of the HL during the early developmental stages. The research has practical contributions regarding the implications for policy and pedagogical decision-makings on HL education.