A method adjusts driving ability of an output buffer. The output buffer has multiple driving ability classes. The method includes the following steps. First, the driving ability of the output buffer is initialized as an initial class among the driving ability classes. Next, a voltage at an output terminal of the output buffer is initialized to an initial voltage. Then, an input voltage is inputted via the input terminal at a first time instant. Next, an output voltage outputted from the output terminal is sampled to obtain a voltage value at a second time instant. Then, whether the voltage value satisfies a predetermined condition is judged. Next, if the voltage value satisfies the predetermined condition, the driving ability class of the output buffer is recorded and set.