Abstract: Based on the Biot-Savart law and principle of vector superposition, an integral method was developed to calculate the magnetostatic field due to irregular coil of rectangular-section conductor. According to conductor shape, the DC coil is divided into two parts: Straight-line current elements and curved current elements, and the latter are fitted by several straight-line current elements. Then, the triple integral for all the straight-line current elements is simplified into simple integral equation to which the numerical solutions are given. To solve the integral equations quickly, some integral equations are derived for the magnetostatic field, including such shapes of current elements as trapezoidal prism, triangle prism and trapezoidal sheet. Numerical results show that the calculated values conform well with the theoretical values.