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The ␥-␥ perturbed angular correlation technique ͑PAC͒ using 99 Ru was applied to investigate the orbital ordering of Ru 4d electrons in Ca 2 RuO 4 . It was confirmed that the ferro-orbital ordering characterized by the double occupation of d xy orbitals at every Ru ion is realized in the stoichiometric Ca 2 RuO 4 . A strong temperature dependence of the quadrupole interaction below the metal-insulator transition around 360 K reflects variation in the occupation of crystal field orbitals enhanced by very pronounced change in the lattice parameters. Temperature dependence of the hyperfine magnetic field H hf at the Ru site shows very steep increase just below the ordering temperature, which could suggest a two-dimensional character of the magnetic ordering in Ca 2 RuO 4 .

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