Microwave spectra of trans and gauche propyl fluoride and its isotopically substituted species have been measured. The r s structures of the trans and gauche isomers of this molecule are determined from the observed moments of inertia. It is found that the CCC angle values are largely different between two isomers, while the CCF angle values stay unchanged. The r s structures of ethyl fluorosilane and ethylmethlyl sulfide are re-examined in order to compare the results with those of propyl fluoride. The differences in the structural parameter values between the rotational isomers are discussed for the present molecules and the analogous molecules such as ethanethiol and ethaneselenol.