An automatic transmission includes six forward speeds and a reverse speed and includes first and second clutches, first, second, and third brakes, and first, second, and third planetary gear sets. An oil pressure control system of the transmission includes a first clutch and second brake controller allowing the first clutch to receive a fastening pressure in fourth, fifth, and sixth speeds, allowing the second brake to receive a fastening pressure in first speed and reverse, and allowing an operation state of the first clutch to be sustained upon operation of the first clutch; a second clutch controller allowing the second clutch to receive a fastening pressure in third and fifth speeds and reverse; a first brake controller allowing the first brake to receive a fastening pressure in second and sixth speeds; and a third brake controller allowing the third brake to receive a fastening pressure in first through fourth speeds.