The phase relations and thermodynamic stabillity of metastable phases in rapidly quenched Ag-Ge alloys have been studied by means of DTA, DSC and X-ray powder diffraction techniques. Solubillity of Ge in Ag is extended from 1.5 at% Ge to 8 at% Ge, when cooling speed of alloy is approximately 104 K/s. Except supersaturated solid solution of Ge in Ag and Ge, there also exists a metastable phase, which has the hexagonal close-packed structure (hep) with lattice parameters a=2.893±0.010 ?, c=4.720±0.010 ?, c/a=1.632. For 20 at% Ge alloy, the activation energy of hep phase transition and supersaturated solution desolvation are 93 kJ/mol and 102 kJ/mol respectively. The nature and transition mechanism of the metastable phases are discussed.