Abstract: Different from the Milky Way, disks of gas-rich galaxies are clumpy. It remains a mystery why they are clumpy and how star formation occurs in these clumps. Molecular clouds constitute a significant fraction of the total mass of galactic disks, and they play important roles in disk dynamics. I will present some recent observational results, and discuss how dynamics of molecular clouds on pc scale influences the dynamics of galactic disk at kpc scale and lead to the formation of giant clumps.