We have numerically studied the formation of frequency comb in a silicon nitride microring resonator with a single-mode waveguide and normal group velocity dispersion (GVD). Various dark pulses such as soliton, breather, and complex soliton have been obtained. The waveform shape in the time domain and the optical spectrum of several dark frequency combs have been presented. Recently, optical microresonator have been studied to be used for wide range of applications. Frequency combs have been initially shown in a resonator with anomalous GVD where the formation of bright soliton(s) and Turing roll is occurred [1-3]. Compared to bright pulses, dark frequency combs in normal-dispersion microresonator have some advantages. First, precise laser detuning is not substantial to overcome the thermal instability issue, which has a vital role in the anomalous-dispersion regime. Furthermore, since the formation of dark pulses is dominantly the result of mode interaction and not the broadband chaotic states, they can be obtained repeatedly in experiment [4]. In order to reach stable dark frequency comb and high order of family-mode interaction, multimode waveguide microresonators have been considered before [4-6]. However, in this study, we have considered a single-mode thin silicon nitride waveguide for the microring resonator (Fig. 1a). The formation of various dark frequency pulses, have been numerically simulated using Lugiato-Lefever model.