Newly formed massive stars can affect the parental molecular cloudsthrough ionization, heating, and expansion of the H II regions, stellarwinds, and supernova-driven shocks. For a better understanding of the
feedback process in the interstellar medium, detailed observationsof the molecular clouds and identification of embedded young stellarobjects (YSOs) are required. In this talk, I will present results ofan extensive observational study of the active star-forming complex
W51. First, I will show the spatial distribution and mass function ofembedded YSOs near the W51 giant molecular cloud over an area of 1.25deg × 1.00 deg selected from Spitzer Space Telescope data. Secondly, Iwill present the CO maps that was observed in the J = 2 - 1 transition
of the 12CO and 13CO molecules with the University of Arizona HeinrichHertz Submillimeter Telescope and compare the molecular cloud morphologywith the distribution of infrared and radio continuum sources. Finally,
I will explain the process of star formation triggered by the expansionof an H II region near W51A.