  This paper investigates instability ranges of a cylindrically symmetric
collapsing stellar object in Brans-Dicke theory of gravity. For this purpose,
we use perturbation approach in the modified field equations as well as
dynamical equations and construct a collapse equation. The collapse equation
with adiabatic index ($\Gamma$) is used to explore the instability ranges of
both isotropic as well as anisotropic fluid in Newtonian and post-Newtonian
approximations. It turns out that the instability ranges depend on the
dynamical variables of collapsing fluid. We conclude that the system always
remains unstable for $0<\Gamma<1$ while $\Gamma>1$ provides instability only
for the special case.
