The magnetic properties of an s = 1/2 Heisenberg triangular antiferromagnet V15 have been studied by proton nuclear magnetic resonance (NMR) at very low temperature down to 100 mK using a He³-He⁴ dilution refrigerator. In total spin S _T = 3/2 ground state above 2.7 Tesla, proton spin-lattice relaxation rate (1/T₁) shows thermally activated behavior as a function of temperature. On the other hand, a temperature independent behavior of 1/T₁ at very low temperatures is observed in frustrated S _T = 1/2 ground state below 2.7 Tesla. Possible origins for the peculiar behavior of 1/T₁ will be discussed in terms of magnetic fluctuations due to spin frustration.