This study aimed to prepare a water-in-oil microemulsion formulation of alendronate. Pseudo-ternary phase diagrams were constructed by using different oils and co-surfactants. The final formulation was decided to be prepared with Captex 200, lecithin, propylene glycol and bidistilled water. Rheological behaviour, phase stability and type of the microemulsion formulation were investigated by Brookfield viscosimeter, centrifugation test and dye method, consequently. Phase behaviour of the formulation was found to be Newtonian. No precipitation was observed in the stressed conditions and formulation was W/O. The physical characterization of the formulation (physical appearance, viscosity, refractive index, conductivity, density and turbidity) was investigated at 4 degrees C and 25 degrees C during 6 months while droplet size was investigated for 3 months. Droplet size of the formulation was between 224-280 nm while viscosity was between 89.9-99.5 mPa.S. The release of alendronate from the microemulsion formulation was examined by dialysis method and found to be 97.2% at the end of 7 h. None of the parameters except refractive index changed significantly during the determined periods. This study succeeded in preparing a stable microemulsion formulation of alendronate.