You can go up to 100% without damaging a C18 column.  Though I presume the THF is pure and subject to peroxide formation (caution peroxides explode!).  But why do you ask?

You will want to eliminate any PEEK tubing in your HPLC system.  Use stainless steel instead.

What are you separating and what total length of each run are you aiming for?

You are doing reverse phase HPLC; C18:C8:C4?

John.

3.5% in water for RP-HPLC! Gradient solvent is usually consisted of acetonitrile/2-propanol equally.

Dear Sunny,

THF can be used ( chromatography grade try Merck) in 20% concentration level provided the solubility of THF in water should be taken care of . Stir and sonicate properly in water.

Secondly THF is volatile the composition of the mobile phase may change take adequate care to ensure no vapour loss.

thanks Bruce Neagle,

THF boiling point is 66 deg C hence column temp of 45 deg C is too high for safe operations.

THF is known to form peroxides which are explosives. 

and my problem is, if am using THF below 15% in mobile phase, resolution between impurity A and B is 1.09. 

thanks  John Morgan,

 Am working with prednisone tablets assay and IDD method by RP-HPLC using L1 column.

You need a resolution factor of 1.5 which is baseline resolution.  Try adding Methanol in increments of ~5% instead of THF.  THF is a very strong organic solvent.

thanks Bruce Neagle,

i tried with methanol 5%, 10% and 15%. But , resolution not changing.