Abstract: The defense responses of Porphyra haitanensis to oligoagars with different degree of polymerizations (DP) were investigated. Oligoagars with three different DPs (agartirose, hexaose and dodecaose) were used to evaluate the effect of structure of oligosaccharides on inducing the defense response in P. haitanensis. After elicited with 100 μg/ml oligoagars, the growing status of P. haitanensis gametophyte was observed by microscope and ultrastructure examination. The impact of the algal response to oligoagars on the epiphytic bacteria was determined by examing the numbers of associated bacteria. The expression level of NADPH oxidase (named Phrboh) was detected by real-time PCR. The concentration of H₂O₂ in the medium was determined by measuring the dimerization of (p-hydroxyphenyl) acetic acid. Results indicated that P. haitanensis responded with an oxidative burst and rapid increases in Phrboh expression when agarotriose and agarohexaose were added to the growth medium. Further, elicitation of P. haitanensis with these two oligoagars caused elimination of epiphytic bacteria. The rot of thallus was lightened and cells of P. haitanensis kept well status in the four days culture. In them, agarohexaose showed stronger effects. Agarhexaose elicited a release of hydrogen peroxide in less than 15 min. The expression of Pyrboh was up-regulated in five minutes and 16.5 fold increases in the expression were observed 5 min after addition of agarohexaose. In contrast, agarododecaose did not have any effect. In summary, P. haitanensis respond immediately after exposure to agarotriose and agarohexaose with a variety of physiological changes, especially agarhexaose could elicit strongest response to P. haitanensis. Oligoagars with high degree of polymerization showed no effect.