A magnetic diagnostic system was designed, fabricated, and calibrated for measuring main parameters of the initial-phase-discharges in Versatile Experiment Spherical Torus (VEST) recently constructed at Seoul National University. Inner and outer Rogowski coils, flux loops, magnetic field probes, and an internal magnetic probe array were designed to have a desired frequency range with a reliable sensitivity. The sensors were carefully shielded to minimize damages from plasmas and microwaves with thermal and electromagnetic protections. The calibration with a known magnetic field generated by a Helmholtz coil or in-situ calibration was performed for the fabricated sensors. The prepared magnetic diagnostics have been successfully used to measure various parameters such as plasma current, loop voltages, and magnetic field structure inside the vacuum vessel for initial plasma experiments in VEST.