Ultra-diffuse galaxies (UDGs) are unusual populations, having luminosities comparable to classical dwarf galaxies but size more like the Milky Way. After the discovery of a sizable UDGs in the Coma cluster, these faint stellar systems have been found in other clusters as well as groups and fields. Although many studies have worked on origins of UDGs, it is still in debate.
I will present results of observational studies for UDGs, including their globular cluster properties. Our results show diversities of UDGs including GC properties, their morphologies, and dynamical masses. We also found the environmental dependence of UDG properties. I will discuss implications of these results for models of UDG formation.