The objective of  this study was to determine the plant density  that result in the best marketable yields and gross crop value per fedan of onion Giza 6 cv. that grown   by sets .Sets were grown at the Experimental Farm of the Faculty of Agriculture, Assuit University, Assuit on September 13 and 15 in 2007/2008 and 2008/2009 seasons, respectively. Sets were grown on rows of 3.5 m long and 50 cm wide. Three rows were included each plot. Sets were grown at 4, 6 and 8 cm between plants (175,116 and 87 plants when sets grown on two sides of row and  262,175and131 plants when sets grown on three sides of row). Results showed  that  , plants gave the smallest bulb diameter when sets were grown at 4 cm space(high density) .The largest space between plants (low density) ,the highest value for both percentage of bolters and doubles were. The highest value of marketable yield was obtained when sets grown at 4 and 6 cm (262 and 175 plants/ row) and on 3 sides per row .Maximum economic returns occurred at higher plant populations.  High incidence of double onions was closely related to low onion plant population. Doubles clearly decreased with increased plant density. Wide plant spacing reduced onion bulb yield, but it significantly increased individual bulb weight and bulb diameter.