In the paper, an attempt to application, known from finance literature The Portfolio Theory and based on it mathematical models has been undertaken. Using Sharp’s single-factor model the beta coefficients for specified crops were calculated. The beta coefficients describe the volatility of a crop in relation to the volatility of the other crops and indicate the possibilities of using production diversification as a strategy of risk management in farms. In the second part of the paper, the Capital Assets Pricing Model (CAPM) was used to estimate the value of expected gross margins, which can to guarantee a positive return from risk cost. The results of CAPM indicates, that in Poland the most popular cereals are underestimated in the terms of risk cost. The diversification of plant production can reduce the total production risk only in a limited range.