Structural and optical characteristics of laterally overgrown GaN pyramids on a (111) Si substrate were investigated by scanning electron microscopy, transmission electron microscopy (TEM), and cathodoluminescence (CL) microscopy and spectroscopy. Cross-sectional TEM images revealed that the threading dislocation density over the window openings is very high, but gradually decreases with increasing GaN thickness, and that dislocations observed over the mask are parallel to the mask interface. Cross-sectional-view CL images taken at different emission wavelengths clearly showed significant differences between the overgrown areas on top of the mask and the coherently grown regions over the windows. A clear reverse contrast in the cross-sectional CL images of bandedge (identical contrast in case of yellow-band) emission was observed by comparing the defect density observed in cross-sectional TEM images. The CL peak intensity ratio of band-edge-emission to yellow luminescence was also investigated as a function of position in the GaN pyramid cross section. It was demonstrated that there exists a strong correlation between structural defects and optical properties in laterally overgrown GaN pyramids on (111) Si substrate.