A catalyst composition for polymerizing a polyolefin having excellent processability and impact strength, a process for producing a polyolefin and a polyolefin resin thereof are disclosed. The catalyst composition comprises at least one first organometallic compound of following formula 1; at least one second organometallic compound of following formula 2; and aluminoxane. The polyolefin resin satisfies following properties (i) to (iv) and (vi), (i) melt flow index (ASTM D1238), measured at 190° C., under a load of 2.16 kg: 0.1 to 1.5 g/10 min, (ii) density: 910 to 930 kg/m3, (iii) the ratio (Mw/Mn), as measured by gel permeation chromatography (GPC): 3.0 to 7.0, (iv) the ratio (Mz/Mw), as measured by GPC: 2.2 to 4.5, and (vi) when the TREF curve of multimodal distribution is deconvoluted, the area of TREF curve having a peak at 50 to 74° C. is 40 to 75% of the total area of the TREF curve.