Precise determination of the double proton transfer tunneling splitting in the ground and vibrationally excited states for FAD is crucial for testing the relevant theoretical models and computation methods as FAD is a building block of chemical and biological complex com-pounds. It became necessary to measure the rotationally resolved spectra of bands near the C-O stretch region of FAD in order to remove the ambiguity of assignment and to investi-gate the perturbations among the vibrationally excited states in detail.Supersonic expansion of molecules was combined to measure spectra of formic acid dimer. High resolution jet-cooled absorption spectra of the formic acid dimer (HCOOH)2(FAD) has been measured in the antisymmetric C-O stretch region at1215-1240cm-1us-ing a rapid-scan tunable diode laser spectrometer. Three vibrational bands of formic acid dimer have been assigned. They consist of the1/22fundamental band and two combination bands. The gap in the spectral coverage in the previous high-resolution study of the middle band [M. Ortlieb and M. Havenith, J. Phys. Chem. A.111,7355(2007)], designated as Band II in this work, was filled. An alternative combinational vibrational band assignment of v23+2v24with symmetry (Bu) was proposed to the first band, Band I.These three vibrational bands were analyzed together using a standard rigid rotor Wat-son A-reduced Hamiltonian in the Ir representation without explicit consideration of the perturbation among the excited vibrational states. Each rovibrational energy level in the ground and vibrationally excited states is split into two levels because of proton tunnel-ing in all the three bands. Effective molecular spectroscopic constants were obtained from least-squares fitting for each tunneling states. The perturbed band-origins in the ascend- ing frequency are1219.71566(15),1225.34593(13), and1233.95817(14) cm-1for Band I, Band Ⅱ, and Band Ⅲ, respectively.The comparison between the observed spectra and simulations from the molecular con-stants shows good agreement near their respective origin bands. The order of the split energy levels is reversed, energetically higher energy levels (Eu) are lower than the ener-getically lower (El) one for all the three bands. That is a clear indication of existence of perturbation among the bands.In addition to formic acid dimer spectra, there were transition lines observed from monomeric spectra. The v5fundamental band of pure formic acid monomer HCOOH was observed. Its perturbed band center was obtained at1220.8cm-1. The same fundamental band for the d2monomer (HCOOD) was observed at1177.09cm-1.