The border effects of oat, yellow lupine and their mixtures were studied to determine the grain yield accuracy based on three year field trails carried out on good ray soil complex. It was found that grain yields are significantly greater in the neighborhood of paths splitting plots than in parts of plot not adjacent to the path. Oat revealed a stronger response to border effect measured by grain yield than yellow lupine, both in pure sowing and in mixtures. Along with the increasing density of oat plants in mixtures with yellow lupine the border effect of its yield escalated. The grain yields that come from entire plots surrounded by paths are overestimated even more than 25%. The error of harvested yields increased as area of plots decreased and their rectangular shape got longer. To avoid the border effects on plant yield, the experimental design should limit the number of paths or leave the adjacent rows to the paths not harvested.

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