Abstract: To study the effects of petroleum hydrocarbon on Ulva prolifera different development stages, the impacts of 0^#diesel oil and crude oil water-soluble components on the adhesion and germination of spores, growth of seedlings, growth and physiological characteristics of U. prolifera thalli at two concentrations were tested in laboratory. The results showed that, for U. prolifera spores, the adhesion was not significantly affected by 3 mg/L crude oil, but could be significantly inhibited by other three oil treatments. Moreover, two high concentration diesel oil and crude oil (12 mg/L) could inhibit the germination of spores, but crude oil at low concentration promoted the early germination process. For U. prolifera seedlings, the growth could be inhibited by diesel oil and high concentration of crude oil. The greatest inhibitory effect on seedlings by 12 mg/L diesel oil cultured on Day 8, which was only 23.69% compared with the control group. For U. prolifera thalli, diesel oil and crude oil only at high concentration showed significant inhibitions on growth, and the relative growth rates were −2.3% and −1.9% in 72 h, respectively. In addition, chlorophyll a, carotenoid and soluble protein of U. prolifera were all inhibited by two high concentration diesel oil and crude oil, while superoxide dismutase and catalase were promoted by these two petroleum hydrocarbons, with the maximum contents increased 24.8% and 210.6%, respectively. In conclusion, as to different development stages of U. prolifera, spore was most sensitive to petroleum hydrocarbon, while adult was the most tolerant. Meantime, the toxic effect of diesel oil on each stage of U. prolifera was higher than crude oil, and the toxicity of high concentration was greater than low concentration.