A method of calibrating ultrasound velocity is provided, including receiving an ultrasound non-delayed data set performing a beam-forming process on the ultrasound non-delayed data set with a plurality of velocities to generate a plurality of aperture images performing a Mean Absolute Percentage Error (MAPE) process to obtain a velocity and two sub-aperture images corresponding to an MAPE value located within an MAPE range finding a first sub-aperture and a second sub-aperture corresponding to the two sub-aperture images, generating a first aperture image and a second aperture image with corresponding velocity performing the MAPE process on the first aperture image and the second aperture image to generate an image error corresponding figure having an error curve finding a trend curve according to the error curve and finding a lowest point of MAPE value on the trend curve and finding a velocity correspondingly to obtain the calibrated ultrasound velocity.
