The hypothesis conditions of green certification model of Vertinsky are extended: assume the production cost of observable quality is negatively correlated to unobservable quality level(environmental friendliness).Main conclusion is,if two firms can freely choose quality level of certification,the firm with high total-quality product should choose to supply high environmental quality;if both firms have to certify their products in terms of government rule,it is socially optimal to let the firm with high total-quality product obtain a high environmental quality certification,however it is uncertain whether one certification level or two different certification levels are socially optimal when both firms certify their products.