Ni-P electroless deposition was performed on a plain carbon steel. Coatings with an average of 8.43%P were obtained and this value was constant throughout the coating cross-section. The coated samples have been heat treated at 320, 400, 500 and 600 degrees C for 30, 60, 90, 120, 150 and 180 min in air and the structural changes induced by heating were related to microhardness. The structural characterization was done by means of X-ray diffraction studies. Vickers microhardness tests were performed on the heat treated samples by varying the load between 0.245-1.962 N (25-200 gf). The results indicate that the maximum hardness was achieved for the samples which have been heat treated at 320 degrees C for 150 min. Similar hardness values have been found for samples heat treated at 400 degrees C for 1 h and it was demonstrated that a further increase in temperature together with an excessive heating time produced a considerable decrease in hardness. Depending on the indentation load, the measured hardness values are a combination between both hardness values of substrate and coating. Several models proposed in the literature have been tested in order to describe the hardness data obtained experimentally.