Direct current (DC) arc discharge method gives high temperature in a short time, which has been widely used to prepare carbon nanotubes. We use this simple approach to synthesize magnesiumoxide (MgO) nanocubes without any catalyst. Structural characterizations indicate that the as-synthesized MgO nanocubes are all single crystalline, and enclosed by {100} facets. The edge lengths mainly range from 60 nm to 100 nm. Most nanocubes interlace with adjacent nanocubes as three types: point contacts, line contacts and face contacts. The blue band in PL spectrum is ascribed to various structural defects, the nanosize effect and surface states of MgO nanocubes.