I will present results inferred from numerical study of geometrically thin accretion disk. The radiation-pressure dominated thin accretion disks undergo thermal instability which has been infamous for decades. We confirmed this instability in a 3D global radiative GRMHD simulation. In order to further understand thermal instability, we simulated GRHD models with \alpha-viscosity and concluded that radiation-pressure dominated thin disk always shows thermal collapse and no evaporation. However, we also argue based on our other global simulations that strong magnetic field could thermally stabilize the disk and show surface accretion. At the end, I shall also talk about time variability present in our GRHD models which show diskoseismic modes, vertical, radial and breathing oscillations. The time-variability found in our simulations could shed light on high-frequency quasi-periodic oscillations (QPOs) observed in low mass black hole X-ray binaries.