Abstract: In June 2016,a total of 81 middle school students were diagnosed with dermatitis of unknown cause and 77 of the diagnosed cases were grade 2 students.The overall attack rate was 4.6%(81/1 753) and maculopapule on the upper arm was the main clinical manifestation.Epidemiological survey showed that practicing volleyball under mango trees was highly associated with the incidence of dermatitis (OR=5.1,95%CI:1.8-14.4);while washing hands after volleyball practice had protective effect against dermatitis (OR=0.4,95%CI:0.2-0.6).Porthesia scintillans larva incased obviously in the mango trees in the school due to warm and wet plum rain season.Students came into contact with the venomous setae of Porthesia scintillans larva which fell to the ground when they practiced volleyball under the trees.No venomous setae of the Porthesia scintillans larva was found directly in affected students,but the survey results indicated that Porthesia scintillans larva was likely to be the cause of the dermatitis.It is suggested that the schools should conduct elimination of Porthesia scintillans larva,especially during their breeding period,and health education in students for the avoidance of any activity under affected trees and washing of hands and exposed skin after activity.