The activity ratio of228Ra/226Ra in leachate from monazite was observed to decrease with increasing pH of the solution, while those of224Ra/ 228Ra, 230Th/232Th and228Th/232Th increase with increasing pH. The leaching behavior of226Ra and224Ra which are formed through 5 decay processes of238U and through 4 decay processes of232Th, respectively, was different from that of228Ra which is formed by a single decay of232Th. Thorium isotopes, 230Th and228Th, decay products of238U and232Th, respectively, were also leached more effectively than232Th. The leaching efficiency of Ra and Th isotopes was estimated to be dependent on the difference in the history of decay processes which have formed the nuclides.