EN
We study the temperature dependence of the in-plane magnetoresistance ρ_{ab}(T) in untwinned YBa_2Cu_3O_{7 - δ} single crystals with the optimal oxygen content and with a small deficit of oxygen atoms at different angles between the external magnetic field 15 kOe and the ab-planes α. We found that at high temperatures in the pseudogap region external magnetic field does not affect the ρ_{ab}(T) but it broadens transitional region T_{c} - T_{c0} from 0.3 K at zero field and α = 0 to approximately 6 K at α = 60° in the field. In case of optimal doping the function ρ_{ab}(T) display a 3D to 2D dimensional crossover when temperature decreases from T_{c} to T_{c0} and scaling near the T_{c0} which we relate to the flux-flow and vortex-lattice melting. In the underdoped sample the vortices are effectively pinned by the random oxygen vacancies and the function ρ_{ab}(T) in transitional region has smooth tails without any traces of crossover and scaling.