What will be the possible conditions to react bromo alkane with diethyl phenylmalonate? How to react 1-bromopentane with diethyl phenylmalonate?
I used NaH (1.5eq) but the yield is less than 30%. I used KOH but yield is low. I used tBuOK and it did not work, I used K2CO3, but not working.
I tried it, K2CO3 2eq with 18-crown-6 and KI 0.1eq, in acetone but starting material is not consumed, almost only 30% has been finished after more than 72 hours and TLC shows a different spot, it does not look like desired product. Starting material is recovering when I use K2CO3, but not in case of NaH 1.5eq with DMF or THF but yield is less than 30%. Toluene with NaH under reflux condition is also behaving like K2CO3 and my next step is hydrolysis and decarboxylation. 
I need guideline about what I am missing here.
You have tried a number of sensible approaches and nothing is working well. I think the problem here is that the anion you are creating is very stable and hindered and no amount of tweaking will significantly improve it. You either accept the yield as it is and scale the reaction accordingly, or change your strategy.

Your next step is hydrolysis and decarboxylation to give a substituted phenylacetic acid so I suggest you start with ethyl phenylacetate. You could alkylate it with NaH (1eq) in DMF with KI then 1-bromopentane. That should alkylate well, but there will be a percentage of di-alkylation so I'm going to suggest you condense ethyl phenylacetate with pentanal and then hydrogenate (or otherwise reduce the double bond). I think this will give you a cleaner reaction mixture and higher yield.