Refractive index control of a polymer film is an important issue in the fabrication of an optical polymer waveguide by using plasma-enhanced polymerization. In a conventional approach, one attempts to control the same by changing the molecular structure by controlling the plasma conditions, for example, the applied power, pressure, and plasma potential. In another approach, elements other than carbon and hydrogen can be incorporated into the film. However, this approach unfortunately causes an increased optical absorption due to excess radicals, e.g. dangling bonds in the film and the unpredictable and undesirable bases produced during the reaction. We have proposed to control the amount of double bonding by varying the source monomer without changing the plasma conditions, where we used C6H6 and C6H10 as the source monomers. In the present paper, we investigate how the refractive index changes by varying the amount of double bonds by controlling the ratio of C6H6 and C6H10 in a mixture. Moreover, we discuss how the CF4 plasma atmosphere affects the refractive index and the molecular structure of the film. Consequently, increasing the ratio of C6H6 in the mixture increases the refractive index of the film linearly. The refractive index decreases until the ratio of CF4 is less than 0.4, and beyond this value the refractive index remains almost constant. Additionally, the deposition rate using CF4 as the atmosphere is much larger than when using Ar. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.