The present invention provides a liquid organomagnesium precursor having formula {Mg(OR′)X}.a{MgX2}.b{Mg(OR′)2}.c{R′OH}, wherein R′ is selected from a hydrocarbon group, X is selected from a halide group, and a:b:c is in range of 0.1-99.8:0.1-99.8:0.1-99.8, and a process for preparing the same. The said process comprises contacting a magnesium source with an organohalide and alcohol in a solvent to form the liquid organomagnesium precursor. The present invention also provides a catalyst system using the organomagnesium precursor and its use thereof for polymerization of olefins. An illustrative example of the claimed process to make the organomagnesium precursor is: At 25° C., magnesium (powder or turnings) is added to organohalide, followed by alcohol in toluene. After heating to 90° C. and keeping there for 6 h the solution became viscous.