Abstract: The convection and microlayer evaporation heat transfer contributions to boiling heat transfer outside a horizontal tube bundle were distinguished by limiting difussion current technique and heat transfer measurements simultaneously.The experimental results show that convection and microlayer evaporation heat transfer coefficients outside a tube bundle increase with increasing heat flux.The enhanced heat transfer increases with decreaseing heat flux.The higher tubes positioned,the higher the contributions of microlayer evaporation and convection to boiling heat transfer outside a tube bundle were.