Grand Unified Theory and Neutrino Mass

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(1) We analyzed the result of recent solar neutrino experiments from the Homestake, Kamioka and Boksan Laboratories. We found that the results from Homestake and Kamioka, if the two data sets are combined, strongly suggest some unusual neutrino properties rather than the modification of astrophysical models of the sun. The neutrino oscillation in matter proposed by Mikheyev and Smirnov among several possibilities yields the most attractive explanation for the neutrino flux defict. The solution suggests that mv_x <similar or equal> 10^<-3>-10^<-4> eV which is in the region predicted by the simple SO(10) grand unified model. We stressed also it is tested by the gallium experiment.(2) In 1985 Simpson reported the evidence for a heavy neutrino with a mass of 17 keV in the beta decay of tritium. Recently, the consistent evidence for this particle has been reported in ^<14>C beta decay spectrum. We first showed that this 17 keV neutrino must be a Dirac fermion. Secondly, We found a natural model which includes one Dirac neutrino and others are Majorana fermions with very small masses. We also showed that there are a class of model in which the 17 kev neutrino is compatible with the MSW-Solution to the solar neutrino problem.