Abstract: The elements composition of United States national ambient air quality standard (NAAQS) and evolution of concentration limits were reviewed. The results shows that NAAQS is divided into two levels in accordance with the protected objects, and composed of the pollutants, averaging time, concentration limits and attainment statistical requirements. Since 1971, it has been revised eleven times. Originally, the pollutants consisted of six kinds of pollutants including SO₂, TSP, Pb, photochemical oxidants (measured by O₃), HC, NO₂ and CO. Afterwards, the pollutants were adjusted by adding PM₁₀, PM_2.5 and Pb and revoking TSP and HC, and there are 7 pollutants currently. The 24-hour average and annual-average concentration limits of PM_2.5 are more stringent than the initial values by 20% and 46%, and the O₃ 8-hour and Pb quarterly average concentration limits are tightened by 6% and 90%, respectively. The attainment statistical requirements have also been adjusted to concentration percentile and annual highest value from the original times above the standard limits.