Probing magnetic fields in the diffuse interstellar medium (ISM) is generally achieved by polarimetry, including stellar polarization and polarized dust thermal emission. Here we present a novel approach, i.e., the Velocity Gradient Technique (VGT), for tracing the magnetic fields. VGT is developed from the fact that magneto-hydrodynamic turbulence is anisotropic in a sub-Alfvenic environment. In this work, we test VGT’s accuracy by applying it to an H I cloud extracted from the HI4PI survey. We compare VGT with the Planck 353GHZ polarization and stellar polarization showing that VGT gives an accurate measurement. We also find Alfven Mach number of the HI cloud is around 0.3 from VGT’s estimation.