Different cropping system and planting patterns of onion and rosemary evaluated to estimate yield advantage and their competitiveness during 2013-14 and 2014-15 growing seasons under irrigated condition at wondo genet Sidama zone, Southern Ethiopia. The experiment comprised of six treatments: sole Onion (250,000 plants ha-1), sole Rosemary (83,333plants ha-1) and four onion-rosemary intercropping mix proportion: 100 onion: 80 rosemary, 100 onion: 60 rosemary, 100 onion: 40 rosemary and 100 onion: 20 rosemary, using randomized complete block design with three replications. Analysis of variance revealed that; intercropping of onion with different population densities of rosemary significantly affected dry bulb yield; highest dry bulb yield was recorded at solitary cropping than that of... intercropped. Similarly; essential oil yield of rosemary significantly influenced by cropping system; highest essential oil yield obtained in sole planted than intercropped. The highest value of land equivalent ratio (1.52), land equivalent coefficient (0.57) and relative crowding coefficient (6.07) obtained when onion intercropped with 80 % rosemary population density. However, positive values of actual yield loss and maximum intercropping advantage obtained in treatments where onion intercropped with rosemary at 20 and 40 % population density. Generally, these finding suggest that intercropping of onion with rosemary at 80 % population density enhanced yield advantage and Competitiveness as indicated by higher land equivalent ratio and relative crowding coefficient. Therefore, the inclusion of onion with 80% a rosemary population density elevated yield advantage and competitiveness over sole planted crop per unit area as indicated by higher LER and relative crowding coefficient.