Heatsinks are essential parts of any thermal management system. High performance heatsinks are required for the cooling systems to be able to manage the ever-increasing power density in electronics and power electronics. The focus of this paper is on the design of high performance naturally-cooled heatsinks with vertical rectangular interrupted fins. A systematic analytical approach is taken, to solve the governing equations of the air flow and heat transfer. Closed-form correlations are presented for temperature and velocity distribution, and an easy-to-use method is introduced to design such heatsinks. Numerical simulations are used to provide better understanding of the physics of flow and heat transfer mechanism. An extensive experimental study is also conducted to verify the results from analytical solution and numerical simulation. Results show that the new-designed heatsinks are capable of dissipating heat up to 5 times more than currently available naturally-cooled heatsinks, with up to 30% less weight. The new heatsinks can increase the capacity of passive thermal management systems significantly. (C) 2015 Elsevier Ltd. All rights reserved.