In this paper, the laws of light-induced degradation (LID) in silicon wafers and solar cells are investigated by using xenon lamp as light source. There are tested 15 types of the silicon wafers contain the including primary wafer, chemical thinned wafer, thermal oxidation passivation wafer, passivation SiNx:H wafer deposited by plasma enhanced chemical vapor deposition, iodine passivation wafers of three different types of silicons: B-doped CZ-Silicon, B-doped Multicrystalline (MC) silicon, and B-doped Upgraded-Metallurgical-grade (UMG) silicon. There are tested 3 types of silicon solar cells: CZ solar cell, MC solar cell, and UMG solar cell. The light intensity is 1000 W/m2 in test. By using WT-2000 tester and solar cells I-V tester, the variations of minority carrier lifetimes of silicon wafers and the I-V characteristic parameters of solar cells with time of light exposure are tested and recorded. Finally the law of LID is found. Under our light condition (light source is a xenon lamp with a light intensity of 1000 W/m2), all kinds of silicon wafers and solar cells are degraded rapidly within the first 60 min, then slowly until the 180 min, finally the rate tends to 0. The LID becomes very slight after 180 min lighting.