An exercise expenditure monitor device and method for monitoring a user's caloric expenditure at exercise, the exercise expenditure monitor device including a heartbeat sensing device for detecting heart rate of the user, a reference storage unit for storing a standard heart rate and a reference heart rate of the user at rest mode, an individual physiology database for storing data of physiology weighted value and a set of individual physiological parameters inputted by the user including sex, age and weight, each set of individual physiological parameters corresponding to a physiology weighted value, a calculating unit which calculates a general caloric expenditure of the user according to an algorithm including dividing the heart rate difference between the exercise heart rate and the reference heart rate by the reference heart rate and multiplying by the physiology weighted value. Preferably, the exercise expenditure monitoring device includes an activity mode selection unit for storing data of activity weighted value and the item of activity that the user takes e.g. walking, jogging, running, jumping, cycling or aerobic dance. Each activity corresponds to an activity weighted value which is provided to the calculating unit which multiplies the general caloric expenditure by the activity weighted value and obtains a true caloric expenditure of the user.