Structural features of a stable hairpin DNA with a tetraloop, dAGGC(TTCG)GCCT (CORE), and its six abasic mutants were investigated by the melting behavior of the UV absorption and circular dichroism spectra. The CORE and an abasic mutant at the second position of the loop exhibit considerable thermodynamic stability and have similar structural features suggesting the same folding with an identical base orientation of the two hairpin DNAs except for the second base in the loop.