An image sensor packaging structure with a predetermined focal length is provided. The image sensor packaging structure includes a substrate, a chip, an optical assembly, and an encapsulation compound. The chip has a sensitization area and is coupled to the substrate. Conductive contacts on the substrate are electrically connected with conductive contacts around the sensitization area. The optical assembly has the predetermined focal length and is disposed above the chip so as to form an air cavity between the optical assembly and the sensitization area of the chip. The encapsulation compound is formed on the substrate to surround the chip and the optical assembly. With the above stated structure, not only can the focus adjusting procedure be dispensed with, but also the image sensor packaging structure can be manufactured by a molding or dispensing process.