The ability of creatine supplementation to increase high intensity performance and to augment adaptations to resistance training, including gains in muscle mass, are well documented.
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The magnitude of gains in muscle mass is variable and the exact mechanism(s) by which creatine promotes muscle fiber hypertrophy have not been identified.

The discovery of myostatin in 1997 was a major breakthrough in understanding the science of muscle growth. Myostatin inhibits muscle growth. For example, experiments in mice that lacked the ability to produce myostatin revealed they had twice as much muscle as normal mice.

This background led researchers to test whether creatine induced increases in muscle mass could be due to down-regulation of myostatin. They had non-resistance trained university students perform 8 weeks of resistance training. One group received placebo and the other creatine following a typical loading period of 1 wk (0.3 grams per kilogram) and then a maintenance dose (0.05 grams per kilogram) for the remaining 7 weeks. Lean body mass increased 4.5 pounds in the placebo group after training indicating the program was very effective alone. The creatine group, however, showed a significantly greater increase in lean body mass of 5.7 pounds. Both groups had a decrease in blood levels of myostatin as a result of training, but the decrease was greater in the creatine group.

The study confirms that creatine supplementation is an effective means to augment gains in lean body mass to training, and provides evidence that this benefit may be partially mediated by lowering levels of the potent inhibitor of muscle growth myostatin.

Reference: Saremi A, Gharakhanloo R, Sharghi S, Gharaati MR, Larijani B, Omidfar K. Effects of oral creatine and resistance training on serum myostatin and GASP-1. Mol Cell Endocrinol. 2009 Dec 22. [Epub ahead of print]