5-Hydroxymethylfurfural (5-HMF) is an endogenous hazardous compound easily generated during thermal processing of foods rich in carbohydrates. Recent research on the generation of 5-HMF in bakery foods generally focuses on western foods such as bread and cookies， while little is known about the level， the formation of 5-HMF and its influencing factors in Chinese desserts. This study analyzed the content of 5-HMF in commercial blueberry-filled pastries and pineapple-filled pastries， and found that its level was significantly higher in the fillings of pastries containing relatively higher amounts of blueberries than in those of pineapple-filled pastries and pastries containing lower amounts of blueberries. Such finding was confirmed by the comparison study of blueberry-filled and pineapple-filled pastries prepared under the same conditions and ingredients other than the fruit incorporated. 5-HMF in blueberry-fillings was 4 times higher than in pineapple fillings， and mostly generated during the stir-frying procedure. The values of water content and pH， as well as the compositional profile of sugar and amino acids in the fillings were compared to explore the possible factors influencing the abundant formation of 5-HMF in blueberry fillings. Firstly， the comparable low pH of blueberry-filling might be one cause of the increased formation of 5-HMF. Secondly， the high amounts of hexoses in blueberry fillings should contribute to the elevated generation of 5-HMF through their dehydration during thermal processing， especially under acidic conditions. Hexoses， i.e. glucose and fructose， were found to be 2.2 times and 3.3 times higher in blueberry fillings than that in pineapple fillings， whereas the predominant sugar in pineapple fillings was sucrose. Moreover， the hydrolysis of sucrose in blueberry fillings was more severe than in pineapple fillings. Thirdly， the amino acids， with serine and glutamic acid being the predominant ones， were higher in blueberry filling than in pineapple fillings， which might further accelerate the formation of 5-HMF through Maillard reactions. The results indicated that the application of blueberry in the thermally processed foods might brought negative impacts by accelerating the endogenous generation of 5-HMF. The current study provides important information and theorical guidelines for the control of the formation of hazardous compounds in traditional Chinese desserts.