Abstract: A double coil system has been designed for measuring the magnetic field of a deflection yoke. A general formula for calculating the field parameters of H0, H2 and H4 is derived when the measuring double coil is in any two positions on x (or y) axis. Even when there exists a small difference o f sensitivities between these two coils, the formula should stili be valid. Measurements are processed by the regression analysis along the x (or y) axis in order to obtain the optimum values for the field parameters. Measurements can be taken for regions farther away from the z-axis.