We establish a theoretical model to analyze the photoassociative (PA) spectroscopy of (3) 3 Σ + state of 85 Rb 133 Cs molecule. The term energy, spin-spin coupling constant and hyperfine interaction constant of nine vibrational levels in (3) 3 Σ + state are determined. Based on the fitted term energy and rotation constant, the potential energy curve of (3) 3 Σ + state is obtained by using RKR method which is compared with the ab initial potential curve. I. INTRODUCTIONPhotoassociation (PA) refers to the transition from the scattering state of two atoms to a molecular excited state by absorbing a photon. PA spectroscopy in ultracold atomic gas possesses very high resolution and reveals properties of both the initial scattering state of the atom pair and the finial molecular states, which are essential for producing ultracold molecules. PA spectroscopy is also used as an sophisticated detection technique to probe the electron Cproton mass ratio variation [1], the number fluctuations in optical lattices [2] and pair correlations in many-body systems [3].PA spectroscopy in ultracold mixture of 85 Rb and 133 Cs atoms has been performed. Since the initial state of PA is the scattering state of the atom pair, and most of the scattering wavefunction lies in the long-range region, the Franck-Condon (FC) factor between the initial scattering state and the weakly bound excited molecular state is expected to be relatively large. Hence, the molecular levels close to the dissociation threshold of the excited electronic states are firstly detected by PA spectroscopy kerman2004production. Some of the weakly bound levels are identified to belong to (2)0 + and (2)0 − states [in Hund's case (c) notation] which are studied in details [4][5][6]. Later, it is shown that the deeply bound levels in the electronically excited states can also be detected by PA spectroscopy [7],although the FC factor is expected to small. These deeply bound levels in the electronically excited states can decay spontaneously to the ground vibrational levels in ground electronic state [8][9][10]. This open up a promising pathway to continuously produce ultracold ground state RbCs molecules via PA followed by spontaneous decay. The deeply bound levels detected in [7] are identified to belong to the Ω = 0 component of the (2) 3 Π state, where Ω is the projection of the total electronic angular momentum on the internuclear axis. Subsequently, the PA spectroscopy of this state is studied extensively [8,[11][12][13][14].Recently, PA spectroscopy of the (3) 3 Σ + state are performed, and several deeply bound vibrational levels, including the ground vibronic level of (3) 3 Σ + state are measured [15]. The PA spectra of the (3) 3 Σ + state have rich hyperfine