An air vent apparatus for a vehicle, which is disposed to supply air into an interior of a vehicle may include a duct housing opened at front and rear sides thereof, at least one blade disposed at a rear side in the duct housing horizontally in a width direction of the vehicle, and having both ends rotatably coupled, a knob mounted on one blade among the at least one blade so as to be slidable in the width direction of the vehicle and a front and rear direction of the vehicle, and adjust an angle of the blade, and a diffuser device mounted in the duct housing at a position spaced apart from a front side of the blade, connected with the knob, adjusting an angle in a left and right direction, and selectively diffusing the supplied air in accordance with a manipulation of the knob.