The myelin sheath in the central nervous system (CNS) provides insulation and nutritional support for axons, facilitating rapid and efficient impulse conduction. Dysregulation of CNS myelination can lead to severe neurological disorders, such as leukodystrophies and multiple sclerosis. Microglia, the resident macrophages in the CNS, play crucial roles in myelin development, though the exact mechanisms are not fully understood. This study was conducted to understand the specific involvement of microglia in CNS myelination. Initial experiments involving the culture of neural stem cells showed that myelin sheath formation only occurred when a microglia-conditioned medium was present. This indicates a critical role of microglia secretomes in in vitro myelination. Moreover, the absence of C1q from the microglia secretomes resulted in myelination failure in the neural stem cell culture. This could be rectified by adding the native human C1q protein, suggesting that C1q is both required and sufficient for in vitro myelination. Further in vivo experiments were carried out to better understand the involvement of C1q in developmental myelination. By mapping C1q expression profiles, the results showed that the spatiotemporal pattern of C1q expression was similar to that of myelin proteins during early brain development. The use of a C1q conditional knockout mouse model (C1qaFL/FL: Cx3cr1CreER) revealed that C1q deficiency prior to the onset of myelination led to a significant reduction in myelin thickness and an increase in the g-ratio. Additionally, C1q deficiency resulted in a less precise alignment between myelin and axons, while the total myelin production remained unchanged. This suggests that C1q may be involved in the association between myelin and axons. In summary, these findings underscore the crucial role of microglia-derived C1q in developmental myelination. This discovery could potentially pave the way for new therapeutic strategies for treating demyelinating diseases.