A 70 mum thick Ni layer was electrodeposited from a sulfamate bath containing various amounts of ammonium ions onto a copper plate. The detailed microstructure of the Ni deposits was characterized using a plane-view and cross-sectional transmission electron microscopy (TEM). The textures of the Ni deposits were also determined using conventional X-ray diffraction. Experimental results indicated that ammonium ions suppressed the lateral growth of Ni deposits and favoured out growth, thus leading to the growth of [1 1 0] and [3 1 0] oriented deposits. A structural refinement effect was observed after ammonium ions were added to the sulfamate bath. Ammonium ions also increased the internal stress and hardness of the deposits. A general Hall-Petch relationship was observed for the dependence of deposit hardness on the average grain size of the Ni deposits. The adsorption of atomic hydrogen and the polar NH3 molecule explains the effect of ammonium ions on the electrocrystallization of Ni.