In　this　work,　a　60-GHz　silicon-based　step-profiled　horn　antenna　is　designed　and　optimized　using　Bayesian　Optimization　(BO).　Specifically,　the　designed　antenna　is　fed　through　a　substrate　integrated　waveguide　(SIW).　The　electromagnetic　wave　is　transformed　upward　by　a　transition　structure　which　is　also　used　for　impedance　matching　between　the　silicon　substrate　and　air.　To　achieve　better　impedance　matching　and　broadside　radiation　gain,　a　step-profiled　horn　is　mounted　on　the　transition　by　stacked　silicon　layers　with　air　cavities　fenced　by　vias.　Before　optimization,　the　impedance　bandwidth　of　the　antenna　is　0.　To　effectively　optimize　this　antenna　that　has　10　parameters　to　optimize,　a　well-designed　objective　function　is　proposed　and　then　BO　is　adopted　to　find　the　optimal　result.　Within　90　times　iteration,　an　optimized　result　satisfying　an　impedance　bandwidth　of　10.4%　is　obtained.　At　the　same　time,　the　simulated　realized　broadside　gain　is　between　5.9　to　6.9　dB　in　the　corresponding　bandwidth.　©　2020　IEEE.