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Mode characteristics of equilateral triangle resonators (ETRs) are analyzed based on the symmetry operation of the point group C3v. The results show that doubly degenerate eigenstates can be reduced to the A1 and A2 representations of C3v, if the longitudinal mode number is a multiple of 6; otherwise, they form the E irreducible representation of C3v. And the one-period length for the mode light ray is half of the perimeter of the ETR. Mode Q-factors are calculated by the finite-difference time-domain (FDTD) technique and compared with those calculated from far-field emission based on the analytical near-field pattern for TE and TM modes. The results show that the far-field emission based on the analytical field distribution can be used to estimate the mode Q-factor, especially for TM modes. FDTD numerical results also show that Q-factor of TE modes reaches maximum value as the longitudinal mode number is a multiple of 7. In addition, photoluminescence spectra and measured Q-factors are presented for fabricated ETR with side lengths of 20 and 30 μm, and the mode wavelength intervals are compared with the analytical results.