  Using the standard infall model of Galactic chemical evolution, we explore
the origin of carbon and calculate the abundance evolution of CNO elements for
8 different models of stellar nucleosynthesis yields. The results show that, in
the early stage of the Galaxy, massive stars are the main producer of carbon,
and that as our Galaxy evolves to the late stage, the longer lived
intermediate- and low-mass stars play an increasingly important role, while at
the same time, metal-rich Wolf-Rayet stars eject a significant amount of carbon
into the ISM by radiative-driven stellar winds. However, from the present
published nucleosynthesis yields we cannot distinguish whether the main source
of carbon in the late Galactic stage is just the massive stars ($M>8M_{\odot}$)
alone, or just the intermediate-, low-mass stars and $M\leq 40M_{\odot}$
massive stars that do not go through the Wolf-Rayet stage. The
$^{12}$C$(\alpha,\gamma)^{16}$O reaction rate is very important in the stellar
nucleosynthesis calculations: a lower rate will give a higher yield of carbon.
The contribution to nitrogen is dominated by intermediate- and low-mass stars,
and the secondary source of massive stars cannot explain the observed [N/Fe] in
metal-poor stars. Most of oxygen is produced by massive stars. The fact that a
higher O abundance in metal-poor stars is derived from the \ion{O}{i}
7771$-$7775 \AA triplet than from the forbidden [\ion{O}{i}] line at 6300 \AA
poses a problem.
