In calculating resonant frequencies of a microstrip resonator the effect of the stray field at resonator edges has been neglected in most publications. The formula for resonant frequencies therefore provided inaccurate results. In the paper the effective dimension and the dynamic dielectic constant of a resonator are defined and obtained for more accurate resonant frequencies and qualty factor by including the effect of stray fields. Theoretical results obtained from the new formula show good agreement with experimental results. In addition, the effect on resonant modes of the distance and coupling angle between the microstrip line and the resonator is also observed.