Properties of light propagating are studied at the interface between right-handed material(RHM) and left-handed material(LHM) according to Fermat's principle,and the leasttime principle is demonstrated unsuitable for the case anymore. The shape equations of single LHM refractive surface are derived on basis of perfect imaging condition. Defects of a LHM slab lens are also pointed out with an improved compound lens is recommended intentionally.