Nanometre-sized tungsten (W) clusters were prepared on MgO(001) single crystalline films using vacuum deposition by burning of a thin W wire. The structure was studied by high-resolution electron microscopy and electron diffraction. The clusters had a b.c.c. structure, but not the A-15 structure which is known as the stable form for small particles of b.c.c. metals. This result is due to the strong interaction between W and MgO(001) substrates. The position of the misfit dislocations at the interface is discussed in relation to the growth centre of the W clusters. Clusters composed of four and five atoms were also detected in the high-resolution images. The image simulation of the clusters suggests that it is possible to determine the adsorption site of W atoms and the inner structure of the clusters from the image contrast.