Vertical-cavity　surface-emitting　lasers　(VCSELs)　have　been　widely　used　in　short-reach　optical　communication　links　and　interconnects　because　of　their　high　modulation　speed　and　low　energy　consumption.　In　this　paper,　the　effect　of　heat　dissipation　on　the　modulation　bandwidth　of　highspeed　850-nm　VCSELs　has　been　investigated.　With　increasing　bottom　distributed　Bragg　reflector　(DBR)　diameter　from　34　mu　m　to　46　mu　m,　optical　output　power　increased　as　well　as　the　-3　dB　modulation　bandwidth.　The　maximum　small-signal　modulation　bandwidth　of　16.2　GHz　was　achieved　with　an　aperture　of　5　mu　m.　Thermal　analysis　showed　that　the　heat　dissipation　of　the　bottom　DBR　diameter,　which　determined　the　differential　gain　of　the　quantum　wells,　has　a　strong　effect　on　the　modulation　bandwidth.