Collisional effects between neutral atoms and plasma particles are not negligible in the ionosphere below 300 km and they are dominant over the Coulomb collisions. Particle simulation method Is dominant over the Coulomb collisions. Particle simulation method is adopted in this thesis to study the sheath formation in the collisional plasmas and the results are compared with that of the collisionless case. Using the ionospheric data obtained by satellites and rockets the floating potential and the sheath region are modeled at the altitudes of 75km and 100km.
