In order to study the effect of periodical weed interference on corn leaf area, a field experiment was conducted at Mazandaran university, Iran in 2006. Eight experimental treatments were arranged in randomized complete block design with three replications. The treatments were represented as weed interference up to 4, 6, 8 and 10-leaf, tasseling, and two weeks after tasseling stages. The weed interference up to harvest and weed-free up to harvest stage treatments were also considered as control. The corn leaf area influenced significantly by periodical weed interference but leaf number per plant was not affected by durational weed interference. Greatest corn leaf area was observed in weed free up to harvest (control) and weed interference up to 4 leaf stage treatments. Increasing weed interference period more than corn 6 leaf stage significantly reduced corn leaf area to lesser than 1 m2 per plant. Long term weed competition changed the corn canopy structure so that the greatest leaf area percentage was attributed to upper levels of canopy. However, leaf area percentage remained constant in lower canopy levels. Results showed that increasing of weed interference period reduced corn leaf area but did not affect leaf numbers per plant and corn compensated this deficiency by changing canopy structure and attributing more leaf area to higher levels of canopy.