Abstract: Agronomic traits of diploid and tetraploid petunias (Petunia hybrida Vilm.) were compared under summer heat in the field. Their physiological characteristics of tolerance to heat stress were studied at 42℃/25℃ (day/night) for 10 days in a diurnal growth incubator. The apparent morphological differences between diploid and tetraploid observed included:(a) the tetraploid plant grew more vigorously; (b) its stem became bigger in diameter; (c) its leaves were denser,thicker,larger and darker green in color; (c) the corolla,calyx,stigma and anther grew larger; and (d) its style and pedicel became longer than the diploid petunias. Under the controlled heat stress,the tetraploid leaves had a lower electrolyte leakage rate and content of malondialdehyde (MDA),but higher contents of soluble sugar,total chlorophyll,proline and SOD activity than in diploid leaves. The results from both the field and the artificially controlled chamber demonstrated that the tetraploid petunia had a higher heat tolerance than its diploid counterpart.