Metformin HCl is an oral anti-diabetic drug used in the treatment of type-2 diabetes but has been linked with gastrointestinal disturbances and lactic acidosis. The aim of this study is to improve the efficacy and achieve dose reduction of metformin HCl by entrapping the drug in solid lipid nanoparticles (SLN). Solid lipid nanoparticles of metformin were prepared by hot homogenization method at different stearic acid and polysorbate 20 ratios. The formulated particles were characterized by particle size, time-dependent pH, differential scanning calorimetry, drug entrapment efficiency (EE) and loading capacity, and in vivo hypoglycaemic activity in mice. P-values less than 0.05 were considered statistically significant. Formulations prepared with lipid-surfactant ratios of 1:5 and 1:2 formed nanoparticles with sizes of 474 and 617 nm, respectively, with moderate entrapment efficiencies and loading capacities, while time-resolved pH showed dependence on surfactant proportion. The size and entrapment efficiency-optimized metformin HCl nanoparticles (1:2 lipid-surfactant ratio) showed faster, higher and sustained reduction of blood glucose in mice than pure metformin HCl powder. Metformin SLN improved the delivery of metformin and may reduce the occurrence of side effects associated with metformin. Solid lipid nanoparticles of metformin HCl may therefore be recommended for effective management of type-2 diabetes mellitus.