The answer seems to be Yes. In this paper, an antibody fragment was used for the purpose.

http://www.ncbi.nlm.nih.gov/pubmed/22483106

Adam is correct in his answer with the original reference he mentions. We have a higher resolution paper as a follow-up to the one he mentions here on a subnanometer structure of an ABC transporter. The Fabs serve as fiducial marks for the cryo em, increase the side of the target and also stabilize a conformational state of the protein that provides insight into the pumping mechanism of the transporter

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4372080/

Thank you Adam, Charles and Tarek!

I have learnt a lot from the papers you gave me. Especially the Fab can be a fiducial marker to help protein acquire a high resolution by Cryo-EM. The most important point is that there's need a rigid conjuction between a protein and tags, which can provide a good sample for Cryo-EM. Fab is specifically rigid interact with the protein, so it's a good choice. However, the conjuction between the protein and tags that can stimulate the solubility and expression of protein, like MBPs, GST and sumo, is often disorder and nonrigid, thus can be a good sample to get the structure of protein by Cyro-EM.

I wanna ask if what I got above is right and exact?

You might want to try imaging the untagged particles in cryo with the use on the Volta Phase Plate, if you have access to it of course. The increased contrast makes imaging of very small particles possible. Currently with the help of phase plate the structures of ~200 kda complexes have been determined at around 4 A resolution. 

Check out the publications:

http://www.ncbi.nlm.nih.gov/pubmed/27563056

http://www.ncbi.nlm.nih.gov/pubmed/26817416

Or you might combine labelling and imaging with the phase plate