This paper presents a compact model of mutual inductance between two planar inductors on the same plane, which is essential to design and optimize multiple output isolated power supply or multi-channel digital isolators. The proposed model considers the influence of shapes, geometric parameters and relative lateral distances of the inductors, and it is derived by solving Neumann's integral. Different planar inductors are simulated by 3-D electromagnetic(EM) software and fabricated on a printed circuit board (PCB) or silicon wafer. The proposed model shows good agreement with simulation and measurement results.