Photosynthesis rate per unit leaf area and leaf longevity are significant components of field cotton performance. The objective of this study was to determine the relationship between photosynthesis rate and age for leaves located at different positions in canopy. Cotton cultivar ‘Sri Sumrong 60' was sowed weekly from June to September 1995 and on May 30, 1996 at Kasetsart University's Suwan experiment station. Net photosynthesis rates at light saturation (P_max) were measured using open system portable photosynthesis system equipped with LED lamp. The relationship between P_max and main stem leaf age, without regard to nodal position, was influenced by plant age. Furthermore, relationships between P_max and leaf age appeared to be similar for main stem leaves on nodes 3-10. For leaves on nodes 12-15, P_max declined at slower rate than for nodes 3-10. For leaves on nodes 16-25, P_max declined much slower than those for nodes 3-15. The main difference of P_max-age relationships of sympodial and monopodial from main stem leaves seemed to be the proportion of leaves that had P_max less than the average for main stem leaf at the same leaf age. At specific leaf age, P_max tended to decline with increase sympodial position. In addition, P_max of monopodial leaves had very large variation, and were less responsive to leaf age than for main stem leaves. Analysis of P_max-node position for specific age revealed that for main stem leaves at lower nodes P_max peaked earlier and longevity was shorter than leaves at higher nodes. Differences in P_max-age relationships for leaves at various main stem, sympodia, and monopodial position were thought to result from leaf acclimation to limiting available PPF resources.