PBA/PMMA　core　shell　latex　was　prepared　with　starve-feed　semi-batch　method.　The　effect　of　the　shell　monomer　feed　rate　and　the　applied　amount　of　emulsifier　on　the　particle　size　and　its　distribution　and　total　number　of　particles　were　studied.　The　evolution　of　particle　size　was　monitored　through　particle　size　analyzer.　And　the　existence　of　core-shell　particles　was　confirmed　through　contrasting　the　theoretical　total　particle　numbers　of　latex　at　different　stages.　The　FT-IR　spectrum　showed　the　core/shell　structure　of　latex　particles　as　well.　The　results　showed　that　emulsifier　played　an　important　role　in　the　formation　of　core/shell　structure.　Homogeneous　polymerization　of　PMMA　and　new　particle　formation　were　favored　at　high　emulsifier.　Hydrophobic　oligomers　of　PMMA　at　low　emulsifier　were　preferentially　adsorbed　on　PBA　core　particles　and　that　resulted　in　formation　of　core　shell　structure.