A hybrid multiphase lattice Boltzmann method is utilized to investigate the vapor-liquid thermos-hydrodynamic behaviors in a flat two-phase thermosiphon. The evaporation regimes in the enclosed narrow space are quantitatively evaluated in a regime diagram with the Bond number and Jakob number, which are directly related to the heat flux, chamber height and filling ratio. The results indicate that the surface evaporation, nucleate boiling, transition boiling and film boiling are sequentially experienced in a flat two-phase thermosiphon as the local heat flux increases (i.e. Jakob number increases). The surface evaporation regime can be subdivided into the pool-surface evaporation and the liquid-bridge evaporation. Liquid-bridge evaporation is a unique evaporation regime due to the confinement of a narrow space under the combined effects of evaporation and condensation. In addition, for a smaller Bond number (i.e. the enclosed narrow space is more confined), the transition boiling and film boiling are more likely to occur, and the interval of Jakob number is smaller for the nucleate boiling in which the thermal performance is superior. (C) 2019 Elsevier Ltd. All rights reserved.