Abstract: It is expounded theoretically that C and m of the dimensionless equation Nu=CRe Pr m,which is in common use,should not be constant.This simple dimensionless equation is unable to reflect accurately the heat transfer when Prandle number equals 0 001～7500.The authors made use of integral equations to solve the laminar flow heat transfer problem of various field flow fields around a plate of Prandtl number 0 001 to 7500.Another form of dimensionless equation is recommended which can describe the functional relationship among Nusselt number,Raynolds number and Prandtl number more accurately.