Same as for UV-vis spectroscopy. A very good practical review:

S. M. Kelly, N. C. Price, Biochim. Biophys. Acta 1997, 1338, 161–185.

Thank you Dr. Eugenio. I will read the suggested paper.

With older instruments, FarUV: 0.02-0.03 mg/ml with 10 mm cuvette, 0.2-0.3 mg/ml with 1 mm cuvette. For near UV measurements increase concentration about 10x. 

With newer instruments (e.g. Chirascan) you can lower concentration 5x or use the concentrations mentioned above for 1mm cuvette with a 0.2 mm cuvette, which is great for reducing buffer interference.

For Far uv CD, 0.2 mg/ml (if pathlength 0.1 cm), and for near uv CD 1mg/ml (if pathlength 1cm) on JASCO-815.

For reference you can see my publication.

Thank you very much Dr. Carlo and Dr. Nida,

Actually, I use 0.25 mg/mL with 1 mm cuvette on JASCO-820, the mean residue ellipticities of my protein at 197 nm and 225 nm are 5000 and -35000 deg cm2 dmol-1, respectively . However the UV absorbances are very high about 3 and no peaks.

I am still learning about CD measurement and I am still confused:

What is the correlation between CD and UV results? actually, I want only to get an information about the triple helical structure of my protein and I have already got it from CD spectrum of my protein.

I am still wondering, can UV spectrum give any important information about the triple helical structure of protein? or it was because the UV absorbances are too high? should I decrease the concentration of the protein to get UV absorbance about 0.2-0.9?

I believe you won't get structural information from UV spectrum. You can dilute your sample to get  UV absorbance  between 0.2-0.9 and them get a good UV spectrum of your protein. The review Dr. Eugenio Vazquez suggest you about CD is a very good one and you can also see Current Protein and Peptide Science, 2000, 1, 349-384. The relationship between absorption and CD spectra depends on the difference in absorption of the left (L) and right (R) circularly polarised components but I don't think you will get any new information from UV spectrum aside from what you already get from CD spectrum.

Thank you Dr. Juliana Fattori. I have a better understanding now.