This study constructs a unique structure of reduced graphene oxide (RGO)/ZnO composite combining starlike ZnO with RGO doped by ZnO nanocrystals through an in situ crystallization method. After mixing graphene oxide (GO) dispersion with Zn(NO) and KOH solution, zinc ions absorbed on graphene oxide sheets transform ZnO nanocrystals, and the free zinc ions form starlike ZnO. After thermal treatment, RGO sheets doped by uniformly dispersed ZnO nanocrystals wrap the starlike ZnO or insert in the gap between them. Composites of different mass ratio of RGO to ZnO (9 : 1, 8 : 1, 5.5 : 1, and 2 : 1) have been achieved by modulating the quantity of Zn(NO) and KOH. The dielectric and microwave absorption properties of the composites show dependence on the mass ratio of RGO to ZnO. The sample with the mass ratio of 8 : 1 shows the most prominent microwave absorption properties, with strong absorption (minimum reflection loss of -77.5 dB) and broad effective absorption bandwidth (6.9 GHz). The enhanced microwave absorption properties can be ascribed to the starlike ZnO, which hinder the agglomeration of RGO, as well as the ZnO nanocrystals, effectively improving the interface of the heterostructure and the impedance matching of RGO.