Structures and electronic states of the HOO radical interacting with water molecules, expressed by HOO(H₂O)_n (n = 1 and 2), have been investigated by means of a direct ab initio molecular dynamics (MD) method. From the static ab initio calculation of HOO–H₂O complex, two types of HOO radical were found: i.e., the HOO radical acts as a hydrogen donor or acceptor in the complex (n = 1). The binding energies of former and latter complexes were calculated to be 8.7 and 3.3 kcal mol⁻¹, respectively, at the QCISD/6-311++G(2d,2p) level. In the case of 1 : 2 complex HOO(H₂O)₂, a cyclic structure with a hydrogen donor of HOO was obtained as the stable form. Effects of zero point vibration on the structures and hyperfine coupling constants of the HOO radical were also investigated. The structures and electronic states of HOO(H₂O)_n (n = 1 and 2) were discussed on the basis of theoretical results.