  We carry out a comparison study on the bar structure in the Illustris-1 and
TNG100 simulations. At $z=0$, 8.9\% of 1232 disc galaxies with stellar mass
$>10^{10.5}M_{\odot}$ in Illustris-1 are barred, while the numbers are 55\% of
1269 in TNG100. The bar fraction as a function of stellar mass in TNG100 agrees
well with the survey $S^4G$. The median redshift of bar formation are $\sim
0.4-0.5$ and $\sim 0.25$ in TNG100 and Illustris-1 respectively. Bar fraction
generally increases with stellar mass and decreases with gas fraction in both
simulations. Barred galaxy had higher gas fraction at high redshift tend to
form bar later. When the bars were formed, the disc gas fractions were mostly
lower than 0.4. The much higher bar fraction in TNG100 probably have benefit
from much lower gas fraction in massive disc galaxies since $z\sim3$, which may
result from the combination of more effective stellar and AGN feedback. The
latter may be the primary factor at $z<2$. Meanwhile, in both simulations,
barred galaxies have higher star formation rate before bar formation, and
stronger AGN feedback all the time than unbarred galaxies. The properties of
dark matter halos hosting massive disc galaxies are similar between two
simulations, and should have minor effect on the different bar frequency. For
individual galaxies under similar halo environment cross two simulations,
different baryonic physics can lead to striking discrepancy on morphology. The
morphology of individual galaxies is subject to combined effects of environment
and internal baryonic physics, and is often not predictable.
