The search for new cooling medium does not limit itself to liquids alone. Liquid-solid suspensions have got a good promise in convective cooling applications. Suspension of common fluids with particles of the order of nanometers (typically 10-100 nm) in size are called 'nano-fluids' which have been found to enhance the heat transfer capability of the base fluid to a considerable extent. With very small volume fraction, such particles are capable of increasing the thermal conductivity and convective heat transfer significantly without the known problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. A recent study on pool boiling on a tube of large diameter (20 mm) shows that the nano-particles degrade the boiling performance with increasing particle concentration pushing up the wall superheat for a given heat flux. The present investigation focuses on an experimental study of pool boiling in water-Al2O3 nano-fluids on horizontal tubes of small diameter. Tubes of small diameter are of interest in efficient cooling applications such as those in electronic modules or LASER devices where miniaturisation is taking place at a rapid pace. However pool boiling of narrow horizontal tubes (4 and 6.5 mm diameter) is qualitatively different from the large diameter tubes due to difference in bubble sliding mechanism. It is found that at the range of narrow tubes the deterioration in performance in boiling is less compared to large industrial tubes which makes it less susceptible to local overheating in convective applications. Thus, the present study on boiling of nano-fluids can act as a guidance for the use of these engineered fluids in the above applications. (C) 2003 Elsevier Ltd. All rights reserved.