The large-size diffraction gratings have become the key unit of some major engineering. All of grating experts around the world are searching the way to produce the large size gratings. This paper reviewed the history of grating manufacturing and introduced the trend of grating development

meanwhile

described the principle of the mosaic gratings. By analyzing the way which foreign country made mosaic gratings

we found the diffraction wave front of every grating is out of phase in a large one and there are a lot of things to do for it to be in-phase. But there are many advantages for manufacturing large-size gratings by mosaic way. We come to a dicision

at present

the way of mosaic gratings is a very effective method for manufacturing large-size gratings.