In this study, we investigate the prompt gamma yields of two carbon ion species, C-11 and C-12, in the therapeutic beam energy range (95MeV u-1 ∼ 300MeV u-1 with regarding to the C-12 beam). For head-to-head comparison, Monte Carlo simulations for mono-energetic pencil incident beams were performed in three energy levels. Beam transportations inside a fixed size water phantom were governed by Geant4 Monte Carlo package and the QGSP-BIC-HP model. Dose distributions, prompt gamma phase space files including energy, time of flight (TOF), position, direction and yield process were recorded. To obtain a dose distribution sensitizer, energy cutoff and TOF windowing were applied to raw prompt gamma data. Depth dose profiles and prompt gamma distributions were then compared. Generally, the absolute yield of the prompt gamma yield per incident C-12 ion is slightly higher than that per C-11 ion. However, the depth distribution of prompt gamma from the C-11 ions seem to be narrower at the Bragg peak region when a tight TOF applied, which may indicate a higher sensitivity to the dose distribution. With regarding to the specific yield processes, prompt gamma from the ion inelastic process is of dominance in both C-11 and C-12 beams. Slightly different depth distributions of gammas from this process was also observed for C-11and C-12. Besides, C-11 has a slight proto This study shows that the C-11 ion beam can provide a comparable prompt gamma yield to, if not better than, the C-12 beam with respect to the dose profile. The C-11 ion beam may serve as a reporter for both prompt gamma imaging and positron emission tomography (PET) for online dose monitoring.