An experimental study of fully developed turbulent air flow in square ducts with two opposite rib-roughened walls was performed to determine the effects of the rib pitch-to-height and rib height-to-equivalent diameter ratios on friction factor and heat transfer coefficients with Reynolds number varied between 7000 and 90,000. Based on the four-sided, smooth-duct correlation and the four-sided, ribbed-duct similarity law, a general prediction method for average friction factor and average Stanton number in rectangular ducts with two smooth and two opposite ribbed walls was developed. There was good agreement between prediction and measurements. The results may be useful for gas turbine blade internal cooling design.