The history, present situation and future of optical frequency standards and optical frequency measurements were presented. The uncertainty of optical frequency standards has come up to 10-11 ~ 10-14. In this way, length unit 'meter' became the derived unit of time unit 'second'. Recently, there was a great breakthrough in this field. The unexpected link of femtosecond mode-locked laser and optical frequency measurement technology make a simple relation between microwave standards and optical frequency standards. Frequency measurements get easier and more accurate because of this relation. This is a new brilliancy in metrology and physics.