Determination of 238Pu in plutonium bearing fuels is required as a part of the chemical quality assurance of nuclear fuels. In addition, the determination of 238Pu is required in nuclear technology for many other applications, e.g., for developing isotope correlations and while using 238Pu as a spike (tracer) in isotope dilution α-spectrometry (IDAS). This determination usually involves the use of α-spectrometry on purified Pu sample. In view of the random errors associated with the counting statistics and the systematic errors due to (1) in-growth of 241Am in purified Pu sample and (2) tail contribution correction methodology in α-spectrometry, the precision and accuracy obtainable by α-spectrometry are limited. Thermal ionization mass spectrometry (TIMS) is generally used for the determination of different Pu isotopes other than 238Pu. This is due to the ubiquitous isobaric interference from 238U at 238Pu in TIMS. Recently, we have carried out studies on the formation of atomic and oxide ions of U and Pu by TIMS and developed a novel approach using interfering element correction methodology to account for the isobaric interference of 238U at 238Pu in TIMS. This methodology is based on the addition of 235U (enrichment >90 atom%) to Pu sample followed by the determination of 238U/235U atom ratio using UO+ ion and determination of Pu isotope ratios using Pu+ ion, from the same filament loading. The TIMS methodology was used for the determination of 238Pu in different Pu samples in U based nuclear fuels from PHWRs with 238Pu content about 0.2 atom%. The 238Pu determination was also carried out using α-spectrometry. This paper reports the results obtained by the two methods and presents the ments and shortcomings of the two approaches.