A constant speed control method for a vehicle on an inclined road allows a constant speed control allowance to be one mile per hour on a flat road and two miles per hour on an inclined surface. If the vehicle speed decelerates two or more times of 1.5 miles per hour or more compared to the preset speed, the vehicle is assumed to be on a slope. A control system is performed by changing the constant speed control allowance from one mile per hour to two miles per hour. If the vehicle speed is within the allowance for 20 seconds, the control system on the inclined road is released. Thus, engine noise according to the frequent change of the T.P.S. value is prevented in advance and frequent gearshifts due to the rapid increment of the T.P.S. value is also prevented, resulting in a stable constant driving speed.