We have successfully used magneto-transport measurement to study magnetization reversal of domain structure in single sub-micron magnetic disks of various sizes, 300nm ≤ diameter ≤ 5μm and 36nm ≤ thickness ≤ 48nm. The magnetoresistance (MR) resulted from the anisotropic magnetoresistance effect can reveals the magnetization reversal of a single disk as confirmed by the magnetic force microscopic results. For a fixed thickness, the remanent domain state can transits from the multi-domain to vortex states by decreasing the disk diameter. Here, the nucleation and annihilation fields of vortex core are determined from disk's hysteretic MR and analytically described by the micromagnetic LLG calculation incorporated with the "rigid" vortex model developed by Guslienko et al. under the consideration of magnetostatic, exchange, and Zeeman energies.