Aux/IAA and auxin response factor (ARF) are two important families that have been well recognized for their roles in auxin-mediated responses. Aux/IAA proteins are short-lived transcriptional regulators that mediate the auxin responses through interaction with ARF transcription factors. Although quite a few members of the Aux/IAA family have been functionally characterized in dicotyledonous plants such as Arabidopsis, but relatively limited information is available in important crops such as rice. This work focused on isolation and characterization of a member of Aux/IAA family in rice named OsIAA1. The results indicated that OsIAA1 was constitutively expressed in all the tissues and organs investigated. The expression of this gene was induced by various phytohormones including IAA, 2,4-D, kinetin, 24-epibrassinolide, and jasmonic acid. Over-expression of OsIAA1 in rice resulted in reduced inhibition of root elongation to auxin treatment, but increased sensitivity to 24-epiBL treatment. In addition, the OsIAA1-overexpression transgenic plants showed distinctive morphological changes such as decreased plant height and loose plant architecture. Protein interaction analysis suggested that OsIAA1 may act through interaction with OsARF1. T-DNA insertion mutant of OsARF1 showed reduced sensitivity to BR treatment, resembling the phenotype of OsIAA1-overexpression plants. In addition, expression patterns of some genes responsive to brassinosteroid and auxin were changed in the OsIAA1-overexpression plants. These data suggested that OsIAA1 may play important roles in the cross-talk of auxin and brassinosteroid signaling pathways and plant morphogenesis.