The　heat　pump　system　with　economizer　coupled　with　scroll　compressor　can　extend　effectively　its　operating　ranges　and　provide　a　technological　method　to　enable　the　heat　pump　to　run　steadily　and　efficiently　in　severe　weather　conditions.　The　intermediate　pressure,　namely　the　working　pressure　of　the　refrigerant　in　the　economizer,　is　an　essential　design　parameter　and　affects　crucially　the　performances　of　the　heat　pump　system.　According　to　the　exergetic　model　setup　for　the　heat　pump　system　based　on　the　second　law　of　thermodynamics,　the　influences　of　the　intermediate　pressure　on　the　performances　are　comprehensively　analyzed　using　experimental　data　of　the　heat　pump　prototype.　It　is　found　that　the　optimal　relative　intermediate　pressure　(RIP)　is　between　1.1　and　1.3.　(c)　2006　Elsevier　Ltd.　All　rights　reserved.