A driving assistant system and method having a warning function for alerting a driver of a risk level, which synthetically determines a running speed of a vehicle, a driver's inattention, a road condition, and the like and provides a warning corresponding to a degree of risk (risk level) to the driver. In particular, a calculated data value corresponding to a degree of a driver's drowsy or careless status is compared to calculation values corresponding to a vehicle speed and the degree of risk of a road to predetermined reference values corresponding to the output values and outputs at least one warning message alerting a driver of risk levels according to the comparison result.