A three-dimensional mathematical model is developed describing arc plasma behavior in double-wire gas metal arc welding (DW-GMAW). The distribution of some physical quantities are completely investigated including arc temperature, arc pressure, velocity, current density, heat flux, magnetic flux density and electromagnetic force by solving conservation equations of mass, momentum, and energy, as well as part of Maxwell's equations. Meanwhile, a comparison study of arc plasma behavior between single-wire gas metal arc welding (SW-GMAW) and DW-GMAW is studied. It is found that due to the inter-attraction of double arcs, the arc plasma deflects to the middle of double welding wires, the maximum arc pressure decreases and the occurrence of welding defects such as undercut and humping is less in DW-GMAW than that in SW-GMAW. In addition, the effect of welding current on inclined angle, maximum temperature of arc plasma and heat flux on workpiece is investigated. The numerical results show good agreement with experimental ones and published literature, which proves that our model has relatively high reliability. The paper lays a foundation for true three-dimensional comprehensive modeling of DW-GMAW including arc plasma, metal transfer and weld pool. (C) 2015 Elsevier Ltd. All rights reserved.