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• Mechanism of heat transfer in in-line elliptical pin-fin micro heat sink is studied. • A numerical model is developed and validated with in-house experimental results. • Cyclic nature of local heat transfer coefficient is observed. • Volume averaged analysis with suitable length scale for each parameter is used. • Correlations describing frictional factor and Nusselt number are proposed. In the present study, hydrothermal characteristics of elliptical pin fin micro heat sink applied with constant heat flux from the bottom are analysed. Experiments are performed on a silicon elliptical pin fin micro sink using de-ionised water as the coolant. A numerical model is developed for single phase incompressible laminar flow and validated against in-house experimental results. The validated numerical model is used to study the effect of geometrical features on the hydrodynamic and thermal characteristics of micro pin fin heat sinks. The behaviour of local heat transfer coefficient is found to be similar after few initial rows of fins from the inlet of heat sink and is dependent on the size of recirculation zone formed behind the fins. A criterion for selecting length scale for defining Reynolds number, Nusselt number and frictional factor is proposed and correlations describing the behaviour of the Poiseuille number and Nusselt number with respect to the Reynolds number and Prandtl number are developed. The detailed understanding and empirical correlations will be useful in design of micro pin fin heat sink with elliptical fins. [ABSTRACT FROM AUTHOR]