A head arm for use in a magnetic disk drive, comprising a guide arm disposed between a magnetic head and an actuator so as to transmit actuating force of the actuator to the magnetic head thereby guiding the magnetic head along a plane of a magnetic disk, a printed circuit fixed to the guide arm, and deflection compensating member for making the coefficients of thermal expansion of said guide arm and said printed circuit substantially equal to each other in at least the direction of seek of the head arm. No thermal deformation in an out-of-plane direction occurs on such a head arm regardless of any variation in the temperature of the environment in which the magnetic disk drive is installed. The head arm may also comprise a guide arm and a printed circuit formed of a moisture absorptive material and covered at all or part of its surfaces with a covering layer formed of a moisture resistive material not permeable to moisture. No deformation in an out-of-plane direction occurs on such a head arm regardless of any variation in the humidity of the environment in which the magnetic disk drive is installed. Therefore, the possibility of occurrence of off-track can be minimized.