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Using a modified ABCD -matrix approach accounting for nonlinear refraction in active medium, we determined the ranges of cavity parameters that provide a mode-locking of Yb: KYW-laser in usual z-fold cavity configuration. Taking the cavity parameters that provide a most efficient mode locking and based on fluctuation model, we performed a numerical simulation of laser operation. We used for our calculations the side-band pump power of 6 W at 982 nm with 1 cm x 50 tmbeam cross section in active medium and the length of 1cm for KYW crystal. Calculations showed that self-starting operation is possible with these parameters and dispersion compensation allows for bandwidth-limited ultrashort pulse generation. The shortest pulse duration was determined to be approximately 200 fs with self-starting build-up time of 130 jts. Such a built-up time is comparable and even shorter than that one for the lasers with semiconductor saturable absorbers. The region of negative dispersion provided by prism pair where a stable ultrashort pulse generation takes place was determined to be (-17000 --42000) fs2.