Eighty-four castrated boars including Laiwu Black (LW) (weight 30-90 kg, n=6) and Lulai Black (LL) (weight 40-100 kg, n=6) were used to study the developmental changes of collagen type III alpha 1 (Col3a1) mRNA expression in the muscle and their association with intramuscular collagen (IMC). The muscle total RNA was extracted to determine the abundance of Col3a1 mRNA using relative quantitative RT-PCR with beta-actin mRNA as the internal standard. The results indicated that the developmental patterns of muscle Col3a1 mRNA in LW and LL pigs were similar. The abundance of Col3a1 mRNA increased with body weight, but decreased a little at 70 kg and 80 kg phases for LW and LL, respectively. On the whole, the expression level of Col3a1 mRNA in muscle of LW was higher than that of LL (P<0.05). Correlation analysis showed that the expression of Col3a1 mRNA in muscle was positively correlated with total and insoluble IMC, but was negatively correlated with IMC solubility for LW pigs (P<0.01) and LL pigs (P<0.05), respectively. These results suggest that the muscle Col3a1 gene expression is affected by body weight and genotype and has important effect on IMC content and characteristics.