In order to find out the importance of flag leaf in Hordeum vulgare L. cv. Hordea in some physiologic traits and photosynthetic and leaf senescence-related gene expression, a field experiment was carried out in Warwick University Research Farm, UK, in 2003. For more accuracy and statistical comparison of the calculated means, the experiment was carried out in 4 replicates. Random leaf samples were taken in 3 periodic times during the plant growth. The studied traits included chlorophyll a, b content, net CO2 assimilation, Fv/Fm ratio, cellular oxidative level (TBARM) and photosynthetic and senescence-related gene expression. Chlorophyll content showed a decreasing trend during the sampling periods. This was greater in other leaves than flag leaf. Net CO2 assimilation rate in flag leaf was greater and more stable than other leaves during the sampling periods. TBARM content went up very fast by progression of leaf senescence. Photosynthetic genes RCBS, LhcII and Cab showed similar expression pattern and generally decreased by by progression of leaf senescence, especially in other leaves. In genes related to leaf senescence, there was notable variety. Expression of MTI and CATI showed relatively similar pattern, whereas SODI and SAG12 showed different expression. In conclusion, results showed that flag leaf had superior role in increasing grain yield. This was due to stability of photosynthesis, chlorophyll content as well as less oxidative damage during flag-leaf senescence, as compared to other leaves. In this respect, activity of photosynthesis genes and homeostatic senescence-related genes significantly was expressed during flag-leaf senescence.