  In this paper we study the footprint of cosmic string as the topological
defects in the very early universe on the cosmic microwave background
radiation. We develop the method of level crossing analysis in the context of
the well-known Kaiser-Stebbins phenomenon for exploring the signature of cosmic
strings. We simulate a Gaussian map by using the best fit parameter given by
WMAP-7 and then superimpose cosmic strings effects on it as an incoherent and
active fluctuations. In order to investigate the capability of our method to
detect the cosmic strings for the various values of tension, $G\mu$, a
simulated pure Gaussian map is compared with that of including cosmic strings.
Based on the level crossing analysis, the superimposed cosmic string with
$G\mu\gtrsim 4\times 10^{-9}$ in the simulated map without instrumental noise
and the resolution $R=1'$ could be detected. In the presence of anticipated
instrumental noise the lower bound increases just up to $G\mu\gtrsim 5.8\times
10^{-9}$.
