The glutathione-Sepharose 4B instruction booklet has this to say:

Pack the gel in any suitable Pharmacia chromatography column.

•Wash the column with 5-10 bed volumes of PBS (140 mM NaCl,2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4) to remove the

preservative.

•Equilibrate the gel bed with 3 bed volumes PBS.

•If needed, clarify the sample by centrifugation and filter thesupernatant through a 0.45 μm filter before applying to thecolumn. The sample may be diluted in PBS + 1% TritonX-100,if too concentrated.

•Apply the sample to the column and discard the eluent.

•Wash the column with 5-10 bed volumes of PBS.

•Elute the bound material with 5 bed volumes of elution buffer

(10 mM Glutathione in 50 mM Tris-HCl pH 8.0) and collect the

fractions.

source: http://www.ritsumei.ac.jp/~h-matsu/GSTmanual.pdf

I suppose that this is why people usually report using that elution buffer. But notice that the equilibration buffer (PBS) contains 140 mM NaCl. So there doesn't seem to be any reason to avoid using a bit of NaCl in your buffer if you think it will help keep your protein from aggregating.

Hi Chen, you probably meant to say that your column was packed using Glutathione Sepharose 4B, not plain Sepharose 4B.

Anyway, if your protein is prone to aggregation, it could be sitting on the top of your column.

There's no way 20 mM NaCl would hurt.  I'd be much more worried about your 10% glycerol.  That might give you some very uncomfortable back pressure and slow down your binding kinetics.  Try to decrease that and running it real slow, maybe, good luck. 

Dear all, thanks for all suggestions. Yesterday I just got my protein purified. I finally changed my buffer to 5% glycerol, 20mM NaCl, 50mM Tris pH8.0 and 10mM reduced Glutathione.

Still I found majority of my proteins were aggregated, but I got some good. Hopefully they are still functional for my next step.