The present invention relates to a method and a circuit of controlling a speed of a brushless motor of a ceiling fan. The circuit includes a motor PWM duty consumption sampling unit and a motor speed sampling to sense a PWM duty and a speed of the brushless motor. A central processing unit is provided to compare the PWM duty and the speed of the brushless motor to a preset maximum PWM duty and a preset maximum speed. When the PWM duty reaches to the preset maximum PWM duty first, the central processing unit sets the current speed to be a maximum speed, and speeds of each level are calculated according to the maximum speed. If the speed reaches to the preset maximum speed first, a constant-speed control will take over to control the brushless motor, and speeds of each level are according to the preset speeds.