We investigate edge and screw dislocation core structures and Peierls stresses of BCC iron using microscopic phase-field (MPF) modeling. Parameters needed for the MPF modeling, such as the generalized-stacking-fault (GSF) energy, are determined based on first principles density functional theory (DFT) calculations. Screw dislocation core has six-fold symmetric structure and 0.05 nm width. The edge dislocation core is three times wider than the screw dislocation core. The Peierls stresses of edge and screw dislocations are estimated as 0.07 GPa and 2.8 GPa, respectively.