The idea is that even ss nucleic acids will form sufficient secondary structures to trap sybrgreen or etbr. That said, I read somewhere that sybr green ii is more suitable to the purpose.

Ethium bromide in gel is good for this. Now-a-days SYBR is used in many cases because of its potency towards the process you mentioned.

So far, I can not visualize ssRNA on agarose gel by using both Etbr and SYBRsafe DNA staining. I do isothermal RNA amplification (NASBA method) and then check the result by electrophoresist. NASBA use DMSO and Sorbitol. I think it may cause there are no secondary structure in RNA, thus EtBr and and SYBR safe did not work with me.

I have read about SYBR green II, i agree with you Tarik. I also found another SYBR, SYBR gold. i read this one is also good for ssRNA. 

Thank u Shankhadeep, Tarik, Tyler

Why arnt you going for etbr or acridine orange... SYBR green can be used buy is quite tricky to detect

SYBR Gold can stain ds and ss. There shouldn't be any issue using it. Even if you don't have secondary structure I do believe you should still be able to see the band. Why are you using agarose (that might be the issue)? which concentration of agarose and ssRNA?  Why don't use polyacrilammide? How long is your ssRNA? If you don't have any loop and you want to be absolutely certain you have your ssRNA you could think to use capillary electrophoresis, if you have any instrument available ( e.g RNA kit Agilent 2100 Bioanalyser).