Despite their large number in the Galaxy, M dwarfs remain elusive objects and the modeling of their photosphere has long remained a challenge (molecular opacities, dust cloud formation). Our objectives are to validate the BT-Settl model atmospheres, update the M dwarf T_eff_-spectral type relation, and find the atmospheric parameters of the stars in our sample. We compare two samples of optical spectra covering the whole M dwarf sequence with the most recent BT-Settl synthetic spectra and use a {chi}^2^ minimization technique to determine Teff. The first sample consists of 97 low-resolution spectra obtained with New Technology Telescope (NTT) at La Silla Observatory. The second sample contains 55 medium-resolution spectra obtained at the Siding Spring Observatory (SSO). The spectral typing is realized by comparison with already classified M dwarfs.