A Ni catalytic probe was used to determine the density of oxygen atoms in a groundbased atomic oxygen simulation facility. The atomic oxygen (AO) source is a low oxygen pressure ionized inductively coupled microwave ECR plasma. A virgin nickel disk was exposed to atomic oxygen environment for 30 min for full activation to form a layer NiO. After successful activation, the temperature of the nickel probe during exposures to atomic oxygen was measured at different experimental conditions. The density of atomic oxygen is studied as a function of oxygen pressure and axis position of the catalytic probe in the vacuum chamber. It was found that the density of AO varies from similar to3x 10(16) to 2x 10(17) atom/m(3) as the plasma pressure was varied from 10(-3)-10(-2) Pa. The density of AO measured by erosion of Kapton is in good agreement with results obtained from the nickel catalytic probe.