The potential of using nanometallic wires inside a matrix as new generation of thermal interface material led us to study the thermal diffusivity of nickel nanowires embedded inside porous alumina template. Thermal diffusivity measurements using a laser flash method showed size dependence for nickel nanowires inside nanochannel alumina (NCA) templates having nominal pore diameters of 200, 100, and 20 nm. Nickel nanowires embedded inside these templates showed decreasing diffusivity values of 10.7x10(-6), 8.5x10(-6), and 6.5x10(-6) m(2) s(-1) at 300 K with decreasing wire diameter when deposited at 40 degrees C. Nanowires fabricated at 60 degrees C showed similar decreasing diffusivity with wire diameter, and a further 42%-48% reduction was observed when compared to 40 degrees C samples. The modified effective medium theory (MEMT) was employed to evaluate the experimental thermal diffusivity. Calculations based on MEMT resulted in mean thermal conductivities of 70.7 and 36.2 W m(-1) K-1 for nickel nanowires fabricated at 40 and 60 degrees C respectively. These values are similar to 20% and 60% lower than the thermal conductivity value of bulk nickel. A strong grain size dependence of thermal diffusivity in the nanowires was observed. It is believed that the decrease in diffusivity in lower temperature wires is associated with defects/dislocations in large single crystals and reduction in wire diameters according to pore diameters of NCA. Whereas in higher temperature wires, the drastic reduction in diffusivity is believed to arise from self-similar fractal morphology composed of nanogranules, close to the dimension of electron mean free path. (C) 2008 American Institute of Physics..