Hello Jitendra,

This may be due to the colored solution of your extract. Probably you get the higher absorbance with high concentration of your extract. Try to use low conc. of extract or use any suitable quencher.

You mau check a titration curve by adding prgressive amounts of plant extract to DPPH solution, and relate it to a standandard quencher like cysteine clorhydrate, as in http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3551182/

In antioxidant assay, such as DPPH assay and Hydrogen peroxide assay, the reagent like DPPH and  Hydrogen peroxide can be reduced by the reductases in plant extract.

So it is normal, when you get a result like that.

One reason maybe you test sample contain less antioxidant than control sample, otherwise the problem maybe the solvent you using or the way you prepare sample.

I suggest you using vitamin E or some other antioxidant as a positive control to check which kinds of issues you encountered.