Abstract: The purpose of this study was to evaluate the yielding performance of F1 single maize hybrids and their mechanical mixture in three plant densities and in two different years. Experiments were conducted in the farm of TEI of Larissa in 2000 and 2001. The genetic material used was consisted of commercial and experimental F1 single-cross maize hybrids and their balanced mechanical mixture for each year. It was found that there was a tension for increasing field yields of almost all hybrids when the plant density was increasing. This was very clear for year 2000, but in 2001 this was found only for the middle density in comparison to the low density because of the presence of common smut. Only hybrid Dias was the exception, with decreasing field yield when the plant density was increasing. The increasing plant density resulted in increasing CV values and number of barren plants. The performance of the mechanical mixture of all hybrids was similar to the mean performance of the hybrids when grown separately. This kind of performance was rendered to the genetic background of modern hybrids, in a way that under stress conditions (allocompetition, density effects) they express stable performance. In general, modern commercial maize hybrids increase field yields under increasing plant density and they can be used as a mixture without decreasing yielding performance. It is possible that allocompetition is not a stronger stress factor than isocompetition in modern maize hybrids.