We consider one of the most powerful and long du ra tion flares of the red dwarf YZ CMi which was revealed from observations in the U band at the Peak Terskol Observatory on 9 February 2008. The stellar luminos ity was increased to the flare maximum by a factor of180 during 30 s and the total flare duration was equal to 1 h. Using the wavelet analysis, quasi-periodical pulsations with a period of about 11 s, an initial modulation depth of 5.5 %, and an exponential time of damping of 29 s are revealed. It is assumed that observed pulsations were caused by fast magnetohydrodynamic oscillations of the flare loop. Based on coronal seismology methods, the following parameters offlare plasma were determined: number density (2·10¹⁰ сm^-3), plasma temperature (3·10⁷ K), and the magnetic field value (0.015 T) in the region offlare energy release.