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Nanocrystalline (NC) Ni-P alloy samples with average grain sizes ranging from a Sew to 127 nm have been produced by crystallizing the amorphous Ni-P alloy at various annealing temperatures. The linear thermal expansion coefficients (alpha(I)) of these NC samples were accurately measured in a temperature range of 300-400K. Experimental results showed that alpha(L) is heavily dependent on the grain size and decreases from about 20.7x10(-6) to 15.5x10(-6) K-1 with an increase in the average grain size from 7.5 to 127 nm. From the alpha(L) dependence of grain size in these NC samples and the at value of the as-cast coarse-grained polycrystalline Ni-P sample we deduced that the difference in thermal expansion coefficients of the interfaces and ofthe nm-crystallites decreases with a reduction of grain size.