The effect of permanent magnetic field up to 12.7 kOe on temperature dependences of conductivity of aluminium-doped YBaCuO single crystals with the system of unidirectional twin boundaries is investigated. In the basal ab-plane the temperature
dependences excess paraconductivity and the field dependence of coherent length xс(0,Н) along the c
axis are obtained. It is shown that the twin boundaries are the effective scattering centers of fluctuation carriers. The temperature dependences of excess paraconductivity is interpreted in ternis of the Hikami–Larkin theoretical
model for layered superconductivity systems. The reasons for suppression of three-dimensional superconductivity fluctuations and nonmonotonic dependence xс(0,Н) in weak magnetic fields directed along the axis c are discussed.