AbstractWe investigate the properties of Lyman Break Galaxies at z ~ 3 in the Spitzer First Look Survey field. The effective area coverage of the study is ~2.63 deg², the largest among similar studies of R ~ 24 mag u-dropouts. Photometric redshifts, ages, dust extinctions and stellar masses of the LBGs have been derived through SED fitting. More than 70% of IRAC-detected LBGs have stellar masses of order of ~10¹¹ M_⊙, comparable with L* galaxies. The MIR luminosity, which represents the rest-frame NIR, is found to be a good tracer of the stellar mass. As the red (optical−MIR) colors of the massive LBGs suggest, IRAC LBGs are older (〈t〉 ~ 0.9 Gyr) and dustier (〈E(B − V)〉 ~ 0.33) than typical LBGs. The age and the amount of dust indicate that the IRAC LBGs are intermediate population between blue LBG population and red DRG population. The number density of LBGs more massive than 10¹¹ M_⊙ is Φ = (1.05 ± 0.15) × 10⁻⁵ Mpc⁻³. This value exceeds the predictions of hierarchicl galaxy formation models.