A brightness controlling method for an all-in-one computer is provided. The all-in-one computer includes an on-screen display adjusting unit having a first brightness value, an operating system having a first system brightness value, a control unit and a scaler. The brightness controlling method includes the following steps. First, the first brightness value is adjusted to a second brightness value or the first system brightness value is adjusted to a second system brightness value. Then, when the first brightness value is adjusted to the second brightness value, the control unit synchronizes the first system brightness value to the second brightness value; when the first system brightness value is adjusted to the second system brightness value, the scaler adjusts the first brightness value to the second system brightness value. The invention also provides an all-in-one computer implementing the brightness controlling method.