A built-in self-test apparatus for a digital-to-analog converter uses a differentiation unit for differentiating a digital-to-analog (DA) signal to obtain the differences between pulses of the analog signal. Next, the analog signal is converted into a digital signal in the light of a threshold voltage by a Schmitt trigger unit. Then, the duty cycles of the digital signal are calculated by a duty cycle retriever, and transmitted into a signature analyzer to calculate the differential non-linearity for error analysis. For processing a high-speed DA signal, the circuit disposed before the differentiation unit may use a test pattern unit, a sample-and-hold circuit and a logic circuit to lower the speed of the DA signal.