A comparative evaluation of ²³⁸Pu determin-ation in NIST-SRM-947-Pu isotopic reference material by alpha spectrometry and thermal ionization mass spectrometry (TIMS) is presented. Electrodeposited sources with geometric progression (G.P.) method to account for tail   contribution were used in alpha spectrometry. In TIMS, the recently developed approach of using ²³⁵U as a monitor isotope in interfering element correction methodology was employed to account for the isobaric interference of ²³⁸U at²³⁸Pu. The atom % abundance of ²³⁸Pu determined by both the methods is in good agreement with each other but is higher than that quoted by NIST in the reference material SRM-947-Pu.