Provided is a valve structure of a shock absorber which is capable of controlling respective damping forces according to a frequency in compression and rebound motions of a piston valve, thereby satisfying both the ride comfort and the control stability. The valve structure of the shock absorber, which has a cylinder filled with a working fluid and a piston rod having one end located inside the cylinder and the other end extending outward from the cylinder, includes: a main piston valve assembly installed at one end of the piston rod and configured to operate in a state that the inside of the cylinder is divided into an upper chamber and a lower chamber, and generate a damping force varying according to a moving speed; and a frequency unit configured to move together with the main piston valve assembly and generate a damping force varying according to a frequency. The frequency unit includes: a hollow housing mounted at a lower end of the piston rod such that the housing is disposed under the main piston valve assembly; and a free piston disposed to be vertically movable within the housing.