The　van　der　Waals　(vdW)　heterostructure,　made　up　of　two　dissimilar　two-dimensional　materials　held　together　by　van　der　Waals　interactions,　has　excellent　electronic　and　optoelectronic　properties　as　it　provides　a　superior　interface　quality　without　the　lattice　mismatch　problem.　Here,　we　report　the　development　and　photoresponse　characteristics　of　a　p-n　diode　based　on　a　stacked　black　phosphorus　(BP)　and　rhenium　disulfide　(ReS2)　heterojunction.　The　heterojunction　showed　a　clear　gate-tunable　rectifying　behavior　similar　to　that　of　the　conventional　p-n　junction　diode.　Under　UV　illumination,　the　BP/ReS2　p-n　diode　displayed　a　high　photoresponsivity　of　4120　A　W-1　and　we　were　able　to　modify　the　photoresponse　properties　by　adjusting　the　back　gate　voltage.　Moreover,　an　investigation　of　various　channel　lengths　yielded　the　highest　photoresponsivity　of　11　811　A　W-1　for　a　BP　length　of　1　mu　m.　These　results　suggested　vdW　2D　materials　to　be　promising　for　developing　advanced　heterojunction　devices　for　nano-optoelectronics.