The time course and regulatory mechanisms of dendritic development are subjects of intense interest. We approached these problems by investigating dendritic morphology of retinal ganglion cells (RGCs) at four early postnatal stages. The RGCs develop from a diffusedly stratified and poorly differentiated group at birth (PO), to sixteen distinct, morphologically well-defined subtypes before eye opening (PI3). Even before bipolar cells make synaptic contacts with the RGCs (P8), most adult-like RGC subtypes are already present. Similar to previous studies in other mammalian species, our results indicate that the initiation of the RGC morphological maturation is independent of light stimulation and of formation of glutamatergic synapses. This study narrowed down the window of RGCs morphological maturation and highlighted a few early postnatal events as potential factors controlling the developmental process. Since mouse is the most popular mammalian model for genetic manipulation, this study provided a foundation for further exploring regulatory mechanisms of RGC dendritic development.