It has been reported that some of electrodeposited alloys are not identical with those represented in thermal equilibrium diagrams; for instance, in the electrodeposition of Ni-Sn alloy system, NiSn metastable phase is formed in a constituent of 40-60 at % of Sn. This study relates to Ni-Sn alloys electrodeposited from the pyrophosphate bath. By the microscopic examination (with T.E.M. and S.E.M.) and measurement of hardness, the following results were obtained on the microstructure and phase of the alloys. These alloys formed a supersaturated solid solution in a range of less than about 10 at % of Sn in Ni, formed a single phase of NiSn metastable phase in a range of about 40-56 at % of Sn, and formed two phases of Ni₃Sn₄ (δ₁) and βSn (ε) in the range of more than about 56 at % of Sn. In the direct observation with T.E.M., the alloys, containing less than about 56 at % of Sn, formed nebulous crystals constituted by microcrystallites of about 50-100Å in size; on the contrary, the alloys containing Sn more than the former, were constituted by many large crystallites of about 300Å in size. The morphological observation of surface with S.E.M. showed that the alloys of less than about 10 at % of Sn had many microcracks, but the alloys of more Sn than that had no microcracks; and the alloys of about 40-56 at % of Sn were very smoothly electroplated. The Ni-Sn alloys forming metastable phases increased in hardness, which was Hv 550-750 at about 50 at % of Sn.