Nonstoichiometric Mg2SiO4 forsterite dielectric ceramics with various Mg/Si ratios (R=2.025, 2.05, and 2.2) were prepared and characterized. The appearance of a MgSiO3 secondary phase could be effectively suppressed by adjusting the nonstoichiometry, and a single-phase forsterite structure was obtained in the present ceramics with Mg/Si ratios of 2.025 and 2.05. The microwave dielectric characteristics were significantly improved in the nonstoichiometric Mg2SiO4 ceramics on eliminating the MgSiO3 secondary phase, where an enhanced Q x f value and a suppressed temperature coefficient of resonant frequency lambda(f) were obtained. The best microwave characteristics were achieved in the nonstoichiometric forsterite ceramics with an Mg/Si ratio of 2.05: epsilon(r)=7.5, Q x f = 114 730 GHz, and lambda(f) = -59 ppm/degrees C.