We present the results of low-resolution optical spectroscopy with OSIRIS/GTC (Optical System for Imaging and Low Resolution Integrated Spectroscopy / Gran Telescopio Canarias) for a sample of ultracool dwarfs. For a subsample of seven objects, based
on 2MASS NIR photometric colours, a photometric spectral type is determined and compared to the results of the optical spectroscopy. For the stars, showing H$alpha$ line in emission, equivalent widths were measured, and the ratio of H$alpha$ to bolometric luminosity were calculated. We find that two dwarfs show the presence of magnetic activity over long periods, LP 326-21 -- quasi-constant-like, and 2MASS J17071830+6439331 -- variable.
