Optical emission spectroscopy (OES) was used to in situ diagnose the CH4-H2-He plasma in order to know the effect of helium on the diamond growth by microwave plasma chemical vapor deposition (MPCVD). The spatial distribution of radicals in the plasma as a function of helium addition was studied. The diamond films deposited in different helium volume fraction were investigated using scanning electron microscope (SEM) and Raman spectroscopy. The results show that the spectra intensity of radicals of Hα, Hβ, Hγ, CH and C2 increases with the increasing of helium volume fraction, especially, that of radical Hα has the most improvement. The spectrum space diagnosis results show that the uniformity of C2, CH radicals in the plasma tends to poor due to the helium addition and resulted in a different thickness along the radial direction The measurement of deposition rate shows that the addition of helium is useful for the improvement of the growth rate of diamond films, due to relative concentration of carbon radicals was increased. The deposition rate increases by 24% when the volume fraction of He was increased from 0 vol.% to 4.7 vol.%. The micrographs of SEM reveal that with the increasing of helium volume fraction, the diamond films’ crystallite orientation changes from (111) to disorder and a twins growth becomes obvious. The secondary nucleation density during growth increases because the high relatively concentration of C2 radicals under higher helium volume fraction (4.7vol. %). In addition, the substrate was etched and sputtered by the plasma, which introduced metallic atoms into the plasma during the deposition of diamond films. Eventually, the existing of secondary nucleation and impurity atoms lead to the appearance of twins and results in the compressive dress.