  A notion of mutation of subcategories in a right triangulated category is
defined in this paper. When (Z,Z) is a D-mutation pair in a right triangulated
category C, the quotient category Z/D carries naturally a right triangulated
structure. More-over, if the right triangulated category satisfies some
reasonable conditions, then the right triangulated quotient category Z/D
becomes a triangulated category. When C is triangulated, our result unifies the
constructions of the quotient triangulated categories by Iyama-Yoshino and by
J{\o}rgensen respectively.
