Provided are magnesium oxide particles which are good in dispersibility in a resin or rubber, can function sufficiently as an acid acceptor or scorch retarder, and do not lower, even after combined with a resin or rubber, material properties thereof; a resin composition; a rubber composition; and a shaped body. The magnesium oxide particles satisfying the following (A) to (C): (A) an average particle size is 5 μm or less; (B) a BET specific surface area is 20 m2/g or more and 200 m2/g or less; and (C) a screen residue by a sieve opening of 45 μm is 0.1% by weight or less.