A review is given on the surface melting of an ice crystal and the physical properties of the quasi-liquid layer (surface melting layer). The thermodynamics of surface covered with the q.1.1. Is first briefly described and then the recent experimental works about the q.1.1. On ice surface are summarized. An ellipsometric study indicates that the critical temperatures of surface melting are -2℃ and -4℃ for {0001} and {101^^-0} faces, respectively, and the temperature dependences of q.1.1. Thickness for both faces are fundamentally different from each other. This result is discussed in conjunction with both the thermodynamic consideration and the structure of ice/quasi-liquid inter-face. On the other hand, the physical properties of q.1.1. Are discussed on the basis of the results obtained by some experimental methods, that is, the ellipsometry, X-ray diffraction, NMR and so on.