Twenty-four male Kazak sheep and 30 Xinjiang fine wool sheep at different ages were selected to investigate the development-dependent expression levels of fatty acid synthase (FAS) gene and hormone-sensitive lipase (HSL) gene in muscle and their effects on the contents of intramuscular fat (IMF). Longissimus dorsal muscle was sampled to measure IMF and total RNA was extracted to determine FAS and HSL mRNA expression levels by real-time PCR. The results showed that: 1) The IMF content increased continuously with growth and showed significant differences (P < 0.05) between different age groups in male Kazak sheep, but in Xinjiang fine wool sheep there was no such difference observed. Furthermore, the IMF contents in Kazak were much higher (P < 0.01) than that of the other breed from day 30 to 90. 2) FAS mRNA expression level was the highest (P < 0.05) on day 0 in Kazak sheep and then declined with growth, in the other breed the gene showed a 'decline-rise-decline-rise' expression manner as the animals grew. HSL mRNA expression level had a similar model in two breeds, in Kazak sheep it was the highest on day 0 (P < 0.05) and in Xinjiang fine wool sheep on day 30 (P < 0.01), then both decreased after this term. 3) In male Kazak sheep, FAS and HSL mRNA expression level were both negatively related to IMF content (r = -0.485 (P = 0.02), r = -0.423 (P = 0.05)), and the ratio of FAS/HSL expression exhibited significantly negatively related IMF contents. In male Xinjiang sheep, there were no obvious relationship between FAS and HSL expression and IMF content (P > 0.05).