For　several　decades,　morphotropic　phase　boundary　(MPB)　in　ferroelectric　materials　has　attracted　constant　interest　due　to　its　great　enhancement　of　piezoelectric　properties.　However,　such　a　MPB　has　been　studied　merely　in　ferroelectric　system,　not　in　ferromagnetic　system.　Recently,　we　reported　the　magnetic　MPB　in　a　ferromagnetic　system　of　TbCo2-DyCo2　and　correspondingly　a　larger　magnetostriction　(i.e.　a　piezoelectricity-like　phenomenon　in　ferromagnets)　occurs　near　MPB.　Very　surprisingly,　such　a　MPB　in　TbCo2-DyCo2　system　has　been　ever　regarded　as　a　spin　reorientation　transition　(SRT)　where　the　spontaneous　magnetization　(M-s)　gradually　rotates　from　＜　111　＞　in　TbCo2-rich　phase　to　＜　001　＞　of　DyCo2-rich　phase　and　vice　versa.　But,　our　experiment　of　synchrotron　x-ray　diffractometry　demonstrates　the　MPB　region　is　the　coexistence　of　TbCo2　and　DyCo2　phases　and　hence　the　M-s　cannot　rotate　gradually　from　＜　111　＞　to　＜　001　＞(1).　Thus,　the　process　of　magnetization　rotation　near　MPB　in　a　ferromagnetic　system　of　TbCo2-DyCo2　remains　obscure　due　to　a　lack　of　in-situ　observation　of　magnetic　moment　rotation.　In　the　present　work,　by　using　a　method　of　Monte　Carlo　simulation,　we　successfully　reproduce　the　observed　effects　and　furthermore　clarify　the　magnetization　rotation　process　near　the　magnetic　MPB　of　TbCo2-DyCo2　systems.　Namely,　the　direction　of　magnetization　changes　discontinuously　from　＜　111　＞　to　＜　001　＞　near　the　MPB　via　the　phase　transition　process　from　TbCo2-rich　phase　to　DyCo2-rich　phase.　Our　simulation　provides　an　effective　way　to　understand　the　origin　of　the　magnetic　MPB.