Square microdisks with round coers are fabricated using a standard GaN-based blue LED on Si substrates.Whispering gallery-like modes in the square microdisks are investigated by finite-difference time-domain simulation.The simulation results reveal that the round coers in square microdisks can substantially suppress the number of light propagation paths and further reduce the number of optical modes.A confocal micro-photoluminescence is performed to analyze the optical properties of the square microdisks at room temperature.The single-mode dominant resonant emission is obtained in the square microdisk with coer radius of 1.5 μm.