Weeds are certainly significant factors in agricultural production, in that they coexist with crops and influence crop production. The detrimental effects of two grass weed species (Avena fatua and Snowdenia polystachya) and two broad leaved weed species (Erucastrum arabicum and Guizotia scabra) at different density levels; 0,10,20,40,80,160 and 320 plants/m2 on grain yield and yield attributes of barley were assessed. Moreover, competitive indices at both low and high weed densities were derived using Cousens's rectangular hyperbola model incorporated in to a GENSTAT program to estimate relative competitiveness of the weed species. The research methodology used and out puts obtained here are confidentially reputable and also will touch the hearts of agricultural researchers, students, policy makers and development workers.