This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G(d)/L-v) values of 0.24-0.80, relative gap width (g/e) values of 0.5-1.5, relative roughness height (e/D) values of 0.022-0.043, relative roughness pitch (P/e) values of 6-12, relative roughness width ratio (W/w) values of 1-10, angle of attack (alpha) range of 30 degrees-75 degrees. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (I) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G(d)/L-v = 0.69, g/e= 1.0, e/D = 0.043, P/e =8, W/w= 6 and alpha=60. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number. (C) 2013 Elsevier Ltd. All rights reserved.