Through　performance　comparison　between　macro-mobility　protocols　and　mirco-mobility　protocols,　it　is　qualitatively　pointed　that　only　when　region　size　increases　to　some　degrees,　the　latter　can　get　better　registration　performance　profits　than　the　former,　but　it　must　pay　extra　packet　transmission　costs,　which　increase　with　the　increase　of　region　size.　The　total　cost　function　was　defined　as　the　sum　of　the　obtained　average　registration　performance　profits　and　the　paid　extra　average　costs　for　packet　transmission　of　the　micro-mobility　protocols　relative　to　the　macro-mobility.　After　that,　an　algorithm　was　presented　to　find　the　optimal　region　size　minimizing　the　total　cost　function,　which　enables　micro-mobility　protocols　to　obtain　higher　gains　with　lower　costs.　Analyses　show　that　when　accessing　router,　the　mobile　host　(MH)＇s　average　residence　time　and　its　average　packet　arrival　rate　decrease,　the　total　cost　value　decreases　and　the　optimal　region　size　increases;　when　the　distance　between　MH＇s　current　network　and　home　network　decreases,　the　total　cost　value　increases.