An analytical model for self phase modulation in Yb-doped fiber laser (YDFL) describing output spectrum and its broadening with increasing power has been developed. Spectral measurements in continuous wave (CW) cladding-pumped YDFL have proved the validity of the model demonstrating hyperbolic secant shape of the spectrum and linear increase of the line width with power in 1-12 W range. At lower powers, spatial hole burning and line self-sweeping effects become important and define the lower limit for the line width.