The present invention provides a method for the preparation of N-(1-oxopentyl)-N-[[2′-(1H-tetra-zol-5-yl)[1,1′-biphenyl]-4-yl]methyl]-L-valine (Valsartan) which comprises; treating N-[[2′-(1-triphenylmethyl-tetra-zol-5-yl)biphenyl-4-yl]methyl]-L-valine methyl ester (X) with oxalic acid or its hydrates in a solvent to produce N-[[2′-(1-triph-enylmethyl-tetrazol-5-yl)biphenyl-4-yl]methy]-L-valine methyl ester oxalate (Xa) and treating the compound (Xa) with a base in a solvent followed by reacting with valeryl chloride in presence of base in a solvent to produce N-[[2′-(1-triphenylmethyl-tetra-zol-5-yl)[1,1′biphenyl]-4-yl]methyl]-N-valeryl-L-valine methyl ester (XI), de-protecting the compound (XI) using anhydrous acidic conditions to produce N-(1-oxopentyl)-N-[[2′-(1-H-tetrazol-5-yl)[1,1′biphenyl]-4-yl]methyl-L-valine methyl ester (V) followed by treating with base in a solvent to produce Valsartan.