Microwave- Optical Frequency Standards at NIM： Realizing of Definition of Metre and SecondLi Tianchu National Institute of Metrology （ NIM ） , Beijing 100013, ChinaAbstract:The paper presents the realization, at NIM, of SI time unit second using the NIM4 cesium fountain clock, SI length unit meter using a femto-second （FS） optical frequency comb and calibration of wavelengths of stabilized lasers by this FS comb. The uncertainty of NIM4 fountain clock was evaluated as 5 × 10^15. The frequency uncertainty of the FS optical comb locked to a H-maser, which is steered by NIM4 clock, was estimated as 2.2 × 10^- 14 The function, significance of Cs fountain clock, stabilized lasers, optical comb and relationship among them are discussed. Finally the progress of NIM5 fountain clock and the construction of a Sr atom optical clock started in 2006 at NIM are reported.[著者文摘]