The present invention relates to a liquid phase additive comprising an alkyl nitrate; a petroleum sulphonates; an aliphatic, aromatic, cyclohexylamines or hetroalkylated lower amines; a hindered phenol based compounds; a phosphate esters and an aliphatic alcohols for use in delayed coking process with decreased coke yield and increased yield of liquid and/or gaseous product and a process for preparing the liquid phase additive. The present invention also relates to a process for thermal cracking of petroleum residue producing petroleum coke and lighter hydrocarbon products by using liquid phase additive.