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NAC (NAM, ATAF1,2, and CUC2) transcription factors play an important role in the responses of plants to various environmental stresses. To investigate the function of SlNAC1, which was found to be a member of the ATAF subfamily in tomato (Solanum lycopersicum L.) plants under heat stress conditions, transgenic tomato plants were generated using an antisense technology. After a treatment at 40 °C for 48 h, in comparison with wild-type (WT) plants, the transgenic plants were severely wilted and exhibited a lower net photosynthetic rate and a maximal photochemical efficiency of photosystem II. Moreover, the transgenic plants displayed a higher ion leakage and malondialdehyde content and a lower proline content. The content of reactive oxygen species (superoxide anion radicals and hydrogen peroxide) were higher, and activities of ascorbate peroxidase and superoxide dismutase lower in the transgenic plants than in the WT plants. The transgenic plants also exhibited a lower accumulation of the transcripts of some heat shock protein genes (Hsp70, Hsp90, sHsp17.4, and sHsp17.6). All of these results suggest that the suppression of SlNAC1 could obviously reduce heat resistance in the tomato plants, and this indicates that SlNAC1 played an important role in the thermal tolerance of the tomato plants.