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The use of rapeseed oil (RO) in tractor engines and other agricultural machinery in its pure form or a mixture of RO with diesel fuel (DF) imposes a number of limitations associated with some difference in physical and chemical properties. Therefore, the most promising is the use of mixed fuel (MF) consisting of DF and RO. The purpose of these studies is to determine the optimal composition of the MF, consisting of DF and RM by optimizing the approximated dependences of the environmental indicators of a diesel engine. To solve this problem, bench tests of the operation of the D-245.5S diesel engine (4ChN 11,0/12,5) were carried out. The following determined environmental performance indicators of a diesel engine are selected: soot (С), nitrogen oxides (NO_x), unburned hydrocarbons (C_xH_y), carbon dioxide (CO₂) and carbon monoxide (CO). The studies were carried out on various compositions of MF, consisting of 80 % DF and 20 % RO, 55 % DF and 45 % RO, 20 % DF and 80 % RO by weight, respectively. As a result of the bench tests, two load characteristics were obtained, the one at a speed of n = 1400 min^–1 corresponding to the value of the maximum torque, and the second at a speed of n = 1800 min^–1 corresponding to the value of the rated power, as well as the external speed characteristic of the D-245.5S tractor diesel engine (4ChN 11,0/12,5). The analysis of the obtained experimental data revealed the dependence of environmental indicators on the rotational speed of the diesel engine crankshaft, the average effective pressure and the addition of RO in MF by weight. Using the least squares method, the approximated mathematical dependences of the ecological indicators of a diesel engine are determined. The analysis of the obtained dependencies showed that: the increase in the crankshaft speed n, the proportion of RO in MF and a decrease in the average effective pressure p_e, leads to a decrease in soot С to 4,0 %, nitrogen oxides NO_x to 100,0 ppm, unburned hydrocarbons C_xH_y to 1,0 ppm, carbon dioxide CO₂ up to 2 % and an increase in carbon monoxide CO up to 0,16 %. As a result of solving the obtained system of equations for the approximated dependences of environmental indicators, the optimal addition of RO to MF of up to 35 % by weight was determined.