A structure of a mask (1) is provided. The structure includes a frame (10), a viewing window assembly (20), and a waterproof skirt assembly (30). The face portion (32) of the waterproof skirt assembly has a circumferential contact area (33). The face portion has a first shape and is made of a material that includes a re-moldable material. When a user (U) puts on the mask through a fastening device (40), the face portion is adapted to be squeezed and deformed to be in a second shape by the user's face, whereby making the circumferential contact area be capable of further following the contour of the user's face; whereas when the user takes off the mask, the face portion remains in the second shape.