The n = 2 mode rotational instability, which appears on a field-reversed configuration plasma produced by a theta pinch, is stabilized by a helical quadrupole field. The critical strength of the field to stabilize the instability is obtained as a function of pitch angle of the helical coil a rad/m. Typically, the plasma in the α = 6 winding field is stabilized by about one-fifth of α = 0 field strength. To physically explain such a good effectiveness of the helical field, the rotation speed of the plasma iso measured by a Doppler shift of a carbon V 2270.9-Å line. However, the clear explanation to the helical effect is not yet given.