Abstract: Natural radioactive decays have been assumed spontaneous．The decays are said to have a constant half-life，despite of experimental and empirical evidence to the contrary．Hsu raised the question if decays are spontaneous，or if they are particle-particle interactions(Hsu K J，1994)．A test of the postulate that beta-decay is a neutrino-neutron interaction has been carried through experimental monitoring of C-14 decay．Our working hypothesis predicted a correlation of the variation of beta-decay rate to the flux of neutrinos．The decay rate of the C-14 sample with its long half-life should be nearly constant during the short duration of the experiment．Our experiments have found，however，rates deviating up to 1% from the theoretical value．The signals of the variation are about 1 000 times greater than the background noise．The variation is not correlative to solar activities，falsifying an assumption that beta decay is caused by collisions of neutrons with solar neutrinos ．There is，however，a positive correlation of the decay rate，during a one and one-half year interval to the intensity of cosmic rays measured at Beijing．The results are consistent with the postulate that beta-decay is activated by neutrinos from cosmic rays ．