An in situ pulse-based delay variation monitor that predicts timing errors caused by process and environmental variations is revealed. The monitor includes a sequential storage device having a mater storage device and a slave storage device, a transition detector that is electrically connected to a node set on an electrical connection pathway from a master storage device to the slave storage device, and a warning signal generator electrically connected to the transition detector. The transition detector receives output of the master storage device to form a warning area by delay buffer, and generates a pulse width output correspondingly according to transition of the data input. Thus the warning signal generator generates a warning signal according to logic action at the pulse width and the clock input when the data input reaches the warning area. Thereby timing errors caused by static process variations and dynamic environmental variations are predicted.