We applied density functional theory calculations to ruthenium nitrosyl complexes, which are known to exist in high-level radioactive waste generating during reprocessing of spent nuclear fuel, to give a theoretical correlation between Ru-99 Mossbauer spectroscopic parameters and ligand field strength for I the first time. The structures of the series of complexes, [Ru(NO)L-5] (L = Br-, Cl-, NH3, CN-), were modeled based on the corresponding single-crystal X-ray coordinates. The comparisons of the geometries and total energies between the different spin states suggested that the singlet spin state of [Ru(II)(NO+)L-5] complexes were the most stable. This result was supported by the benchmark calculations of the Ru-99 Mossbauer isomer shift (delta) and quadrupole splitting (Delta E-Q) values. The calculated results of both the delta and Delta E-Q values reproduced the experimental results by reported previously and increased in the order of L = Br-, Cl-, NH3, CN-. Finally, we estimated the ligand field strength (Delta(o)) based on molecular orbitals, assuming C(4)v, symmetry and showed the increase of A values in that order, being consistent with well-known spectrochemical series of ligands. The increase attributes to the strengthening of the abilities of sigma-donor and pi-acceptor of the L-ligands to the Ru atom, resulting in the increase of the delta values. Furthermore, the increase of the sigma-type donation into Ru d(x2-y2) orbital and the pi-type back-donation from Ru d(xz), d(yz) orbitals in that order caused the decrease of the electron density along the Ru-NO axis, resulting in the increase of the Delta E-Q values. This study is expected to contribute to the ligand design for the ruthenium nitrosyl complexes, leading to the drug design for NO carrier and the decontamination of radioactive ruthenium from the ecological system, as well as the recovery of platinum-group metals from high-level radioactive waste.