Biotic stresses caused by nematodes create restrictions in plant growth. In this article, the physiological effects of root-knot nematode (Meloidogyne incognita  Race 2) stress was assessed on photosynthetic fluorescence attributes in two tomato cultivars, Falat Y as a susceptible, and Gina VF as a tolerant cultivar. The experiment was done in a completely randomized split plot design under greenhouse conditions with four nematode populations, 0 (as control), 500, 1000 and 2000 second stage juveniles, and four sampling times (20, 40, 60 and 80 days after inoculation). After sampling, purification, identification, and population of root-knot nematode species and race were determined, and after amplification of purified population on the tomato cultivar cv. Rutgers, inoculums were sufficiently obtained.  In four-leaf stage of the plant growth, the nematode inoculum levels were introduced, and photosynthetic parameters were evaluated at different times. The results showed that by increasing levels of nematode inoculum, chlorophyll fluorescence parameters were highly affected. In general, the nematode-stressed plants under both light and dark conditions, amount of minimum and maximum fluorescence (F₀ and F_m) and the difference between them (F_V), rate of non-photochemical quenching, photochemical quenching and descriptive parameter of non-chemical quenching increased, while the efficiency rate of photosystem II under both conditions trended to a downward with increasing nematode levels. Thecorrelation between nematode levels and various sampling times had different effects on the measured characteristics. Overall, Falat Y cultivar had relatively greater photosynthetic parameters than Gina VF cultivar.