A Fabry-Perot ytterbium-doped laser, made by a double-clad microstructured optical fibre was designed for high power applications. The air holes extending along the propagation direction were designed to obtain an average cladding refractive index lower than that of the core. The rectangular inner cladding was chosen to enhance the absorption of the pump power. A slope efficiency S = 93.6% and a threshold pump power P-th = 217 mW were calculated for a fibre L = 4.4 m long, Ytterbium concentration of N-Yb = 6 x 10(25) ions/m(3), input min-or reflectivity of R-1 = 0.99 and output reflectivity of R-2 = 0.05. (c) 2005 Published by Elsevier B.V.