Abstract: C. Libove[1]proved that at least one of the halfwave numbers m and n in x and y directions of the buckling mode will be I for simply supported rectangular ort hot ropic plates under biaxial compression. This paper will give the physical conditions of m=1or n=1, and, at the same time, show the way of finding appropriate value of m when n=1 and that of n when m=1 and even lead to explicit expression for m and n. Thus, the buckling mode may be determined completely and the expression of critical load may beformlated explicitly.