The boundary lubrication properties of diamond and surface-modified diamond films were studied with varioussolutions such as supplied water, physiological salt solution and sodium chloride solution. The coefficient of friction between a polished diamond film and a diamond tip was as low as μ=0.01–0.05 under boundary lubrication with water. This result was attributed to the small contact area and low shear strength of water strongly adsorbed on diamond surfaces. The friction coefficient of diamond lubricated with the supplied water was lower than that with distilled water. To clarify the reason, the supplied water was electrolytically dissociated into acid ion water and alkali ion water, using an ion water generation device. The friction coefficient of diamond films lubricated with the acid ion water, which collected at the positive pole, was lower than that with alkali ion water collected at the negative pole. This friction-decreasing effect of the acid ion water and the supplied water is attributed to the present of the negative ions. The friction coefficient of diamond film lubricated with physiological salt solution (0.85%) was lower than that lubricated with distilled water. Furthermore, the boundary lubrication properties evaluated under lubrication with dilute hydrochloric acid (HCI) and sodium hydroxide (NaOH) solution at the same molar concentration. In this case, the friction coefficient when using HCl solution was low, as the same as with physiological salt solution, while the friction coefficient of diamond lubricated with NaOH solution was higher than that in the case of using physiological solution. It was concluded that Cl− ions contained in physiological salt solution and dilute hydrochloric acid contribute to friction reduction at the diamond-diamond interface. The boundary lubrication properties between diamond films and a diamond tip under lubrication with sodium chloride solution were evaluated. The friction coefficient showed a tendency to decrease until a sodium chloride concentration of 0.85%, and thereafter increased with the concentration. The friction coefficient in the case of using 0.85% concentration sodium chloride solution, which is nearly the same as in physiological salt solution, was minimum. The friction coefficients of polished diamond, fluorinated diamond and oxygen-plasma-treated diamond films lubricated with water solutions were low and exhibited similar properties. On the other hand, the friction coefficient of nitrogen-ion-implanted diamond film was higher than those of other diamond films.