Yarn ultimate mechanical properties are important for prediction of fabric mechanical behavior and estimation of yarn complex quality. The prediction of single component cotton yarn strength based on the knowledge of fibrous bundles strength (HVI) and correction factors for fibers orientation and yarn packing density is now at disposal. In the case of two component yarns from cotton /polyester fibers mixtures is common that at some critical mixing ratio the minimal yarn strength appears. The prediction of critical mixing ratio is usually estimated by using of Hamburger model. Main assumptions are linearity of stress strain curves for both components and linear mixing rule. The main aim of this contribution is description of procedure for computation of critical mixing ratio based on the two methods:

1.The experimental values of strengths for set of yarns with various mixing ratios are at disposal. In this case the improved statistical method for estimation of critical mixing ratio is proposed. 2.Experimental information about strengths of both components, yarn fineness and yarn twist are at disposal. In this case the predicted cotton and polyester yarn strengths and break elongations are firstly computed and then the Hamburger model is used. The predicted strengths are computed from special model based on the modification of fiber strength by coefficients reflecting fibers orientation and yarn packing density.

Both methods are compared on the sets of experimental data according to their prediction ability.