In　recent　years,　water　(R718)　as　a　kind　of　natural　refrigerantwhich　is　environmentally-friendly,　safe　and　cheaphas　been　reconsidered　by　scholars.　The　systems　of　using　water　as　the　refrigerant,　such　as　water　vapor　compression　refrigeration　and　heat　pump　systems　run　at　sub-atmospheric　pressure.　So,　the　research　on　water　boiling　heat　transfer　at　sub-atmospheric　pressure　has　been　an　important　issue.　There　are　many　research　papers　on　the　evaporation　of　water,　but　there　is　a　lack　of　data　on　the　characteristics　at　sub-atmospheric　pressures,　especially　lower　than　3　kPa　(the　saturation　temperature　is　24　degrees　C).　In　this　paper,　the　experimental　research　on　water　boiling　heat　transfer　on　a　horizontal　copper　rod　surface　at　1.8-3.3　kPa　is　presented.　Regression　equations　of　the　boiling　heat　transfer　coefficient　are　obtained　based　on　the　experimental　data,　which　are　convenient　for　practical　application.