Abstract: The heat transfer performance of deionized water flowing in the large length/width ratio pin-fin micro channel was experimentally studied with two different ratios of channel to fin. The results show that, when the Reyonolds number is less than 650 and the inlet temperature is fixed at 40 ℃, the channel can achieve a heat dissipation of 21.32 W/cm² and the maximum wall temperature is below 77 ℃. The thermal properties of the channel can be summarized as follows for a fixed Reyonolds number: the wall temperature increases with the heating power and along with the flow direction; the local Nusselt number decreases along with the flow direction, then increases, and gradually tends to a fixed value. In the case of a long pin-fin channel, the entrance channel effect can be neglected, the average Nusselt number is independent with heating power, and only increases with Reyonolds number. Based on these results, a new correlation is proposed to evaluate heat transfer performance of pin-fins, which considers the Nusselt number as a function of the channel/fin ratio, Reyonolds number and Prandtl number.