This study aimed to use co-expression networks to reach a better understanding of the genetic mechanisms underlying marbling development and assess the effect of different diets on the transcriptomic profile and the resulting phenotype. We evaluated development of marbling in Longissimus dorsi muscle extracted from 45 Hanwoo steers at 8, 12, 18, 24 and 30 months of age. The effect of two different feeding conditions (high/low feeding) were evaluated on the gene expression in the relation with the marbling score. In the four groups according to the marbling score (MS) and the feeding treatment: HighFeed_HighMS, HighFeed_LowMS, LowFeed_HighMS, and LowFeed_LowMS we found 818, 928, 899, and 946 co-expressed genes respectively. The activity of modules changed significantly with the age indicating different expression profiles for genes involved in muscle growth (PI3K-Akt signaling, Focal adhesion, ECM-receptor interaction), metabolic regulation (Biosynthesis of amino acids and Glutathione metabolism) and lipid deposition (Fatty acid metabolism, Regulation of lipolysis in adipocytes) through the animal development. The effect of the feeding condition in animals that developed low MS showed the activation of pathways related to stress and maintenance of cell homeostasis under nutritional limitations, while pathways involved in fat deposition and lipid mobilization where observed under high feeding.