The article deals with the transition flow convective heat transfer and friction factor characteristics of water/propylene glycol-based CuO nanofluids flowing in a horizontal circular tube fitted with and without helical inserts. CuO nanoparticles of average particle diameter less than 50nm were suspended in the base fluid and nanofluids of three different concentrations were prepared. Experiments were also conducted by inserting helical inserts having twist ratio in the range of 0 to 9 and Reynolds number ranging from 2500 to 10,000. The Nusselt number obtained with 0.5% concentration of CuO nanofluids is about 28% higher in a plain tube and is increased further up to 5.4times over the base fluid when the helical insert with twist ratio 3 is used. In the plane tube the friction factor is increased by 10% and further enhanced to 140% with the usage of helical insert compared to the friction factor obtained with the base fluid alone. The friction factor penalty is very much less compared to the benefit of heat transfer enhancement. Correlations for predicting Nusselt number and friction factor are developed using the experimental data pertaining to CuO nanofluids in a plain tube with and without the helical inserts.