Extraction behavior of U(VI) species from aqueous HNO3 solution to [Hbet][Tf2N] or [TMPA][Tf2N] phase (Hbet: betainium, TMPA: N,N,N-trimethyl-N-propylammonium, Tf2N: bis(trifluoromethylsulfonyl)imide) has been studied. It was found that the extractability (E) is 62% in the extraction from 0.01 M HNO3 to [Hbet][Tf2N] and decreases with an increase in [HNO3] and that the U(VI) species are not extracted into [TMPA][Tf2N] phase. These results suggest that the extraction of U(VI) species is due to the complex formation of UO22+ with the COO- group of deprotonated betaine (bet). Furthermore, other metal species such as Na(I) and Al(III) in 0.01 M HNO3 solution were found to be hardly extracted.