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.