The present invention provides highly pure repaglinide substantially free of dimer impurity, and process for the preparation thereof. The present invention also relates to 2-ethoxy-N-[(1S)-3-methyl-1-[2-(1-piperidinyl)phenyl]butyl]-4-[2-[[(1S)-3-methyl-1-[2-(1-piperidinyl)phenyl]butyl]amino]-2-oxoethyl]benzamide, an impurity of repaglinide, and a process for preparing and isolating thereof. The present invention further relates to pharmaceutical compositions comprising solid particles of pure repaglinide substantially free of dimer impurity or pharmaceutically acceptable salts thereof, wherein 90 volume-percent of the particles (D90) have a size of less than about 400 microns. The present invention also provides an optical resolution method of racemic 3-methyl-1-(2-piperidino-phenyl)-1-butylamine and use thereof for the preparation of repaglinide.