Abstract: The characteristics of heat transfer and total pressure drop of flow in an internally ribbed rectangular channel were studied experimentally by using two experimental models.The two larger sides of the models were made of ribbed aluminum plates;the two smaller sides were made of bakelite plates and treated as adiabatic surfaces.The acute angle between the ribs and the axis of the channel was defined as the rib angle.For the first experiment model,the rib angles for the two ribbed wall were 45° and -45° respectively,which was known as 45/45 model ,and for the second experiment model,the rib angles were 45° and -60° respectively,named as 45/60 model.All ribbed plates had rib width to pitch ratios t/p of 0.25, rib width to height ratios t/eof 0.3,and rib height to channel height ratios e/H of 0.5.The ribs from each plate of the channel were in contact with each other in their tips,and oriented so as to form a grid pattern.The experimental Reynolds number was in the range of 0.5×10⁵～1.2×10⁵.The experimental results showed that,in comparison to fully developed flow in a smooth pipe of equivalent hydraulic diameter the Nusselt number inside the grid-ribbed rectangular channel was increased up to 5 to 9 times higher,and total pressure drop was enlarged by up to three orders of magnitude.