Abstract: A study was carried out in order to investigate differences between six ecotypes of vetiver grasses (Karnataka,Sunshine, Huffman, Kandy, Monto and Veriveria nermorlis)in photosynthetic physiology and rules of diurnal variation of photosynthetic parameters. Results show that the six ecotypes of vetiver grasses varied sharply in light saturation point (LSP), light compensation point (LCP), maximum net photosynthetic rate (P_n,max), dark respiration rate (R_d) and diurnal variation of net photosynthetic rates (P_n). Among the six ecotypes, Karnataka and Veriveria nermorlis were higher in LSP, LCP and P_n,max; Sunshine and Kandy, lower in LSP; Sunshine and Huffman, lowest in LCP;Monto, lowest in P_n,max; Karnataka, higher in R_d; and Huffman, lower in R_d. Furthermore, the diurnal variations of P_n of Karnataka and Veriveria nermorlis both could be depicted as two single-peak curves with P_n,max appearing at 12:00, while the diurnal variations of P_n of the other four ecotypes, as double-peak curves with an obvious midday depression appearing at 12:00. Relationships of P_n with ecological factors, such as air temperature (T_a), air CO₂ concentration (C_a), relative air humidity (H_R) and photosynthetically active radiation (PAR), were analyzed by correlation, path and decision coefficients, respectively, indicating that PAR is the primary determining factor for P_n of the six ecotypes of vetiver grasses, H_R, the main limiting factor for P_n of Karnataka, Huffman, Monto and Veriveria nermorlis, and T_a the main limiting factor for Pn of Sunshine and Kandy.