  Effects of non-uniform initial mass density and temperature on the plasmoid
instability are studied via 2.5-dimensional resistive magnetohydrodynamic(MHD)
simulations. Our results indicate that the development of the plasmoid
instability is apparently prevented when the initial plasma density at the
center of the current sheet is higher than that in the upstream region. As a
result, the higher the plasma density at the center and the lower the plasma
$\beta$ in the upstream region, the higher the critical Lundquist number needed
for triggering secondary instabilities. When $\beta =0.2$, the critical
Lundquist number is higher than $10^4$. For the same Lundquist number, the
magnetic reconnection rate is lower for the lower plasma $\beta$ case.
Oppositely, when the initial mass density is uniform and the Lundquist number
is low, the magnetic reconnection rate turns out to be higher for the lower
plasma $\beta$ case. For the high Lundquist number case ($>10^4$) with uniform
initial mass density, the magnetic reconnection is not affected by the initial
plasma $\beta$ and the temperature distribution. Our results indicate that the
guide field has a limited impact on the plasmoid instability in resistive MHD.
