The 50 kton INO-ICAL is a proposed underground high energy physics experiment at Theni, India (9 • 57 N , 77 • 16 E) to study the neutrino oscillation parameters using atmospheric neutrinos. The Resistive Plate Chamber (RPC) has been chosen as the active detector element for the ICAL detector. An experimental setup consisting of 12 layers of glass RPCs of size 2 m × 2 m has been built at IICHEP, Madurai to study the long term stability and performance of RPCs which are produced on a large scale in Indian industry. In this paper, the studies on the performance of RPCs are presented along with the angular distribution of muons at Madurai (9 • 56 N, 78 • 00 E and Altitude ≈ 160 m from sea level).The primary cosmic rays originating from outer space mostly consist of high energy protons and a smaller fraction of other high Z-nuclei. The interaction of primary cosmic rays with the air molecules in the earth's atmosphere results in showers of secondary particles, which are mostly composed of pions (π + , π − and π 0 ), in the upper atmosphere. The neutral pion mainly decays via electro-magnetic interaction and produces 2γ while charged pions decay via the weak interaction leading to muons and neutrinos, π + → µ + + ν µ and π − → µ − + ν µ . Muons then decay through, µ + → e + + ν µ + ν e and µ − → e − +ν µ +ν e . INO will be an underground experiment to precisely measure neutrino oscillation parameters using signature of neutrino interactions. It will also determine the sign of the 2-3 masssquared difference, ∆m 2 32 (= m 2 3 − m 2 2 ) through matter effects, the value of the leptonic CP phase and, last but not the least, the search for any non-standard effect beyond neutrino oscillations [1]. About 28000 glass Resistive Plate Chamber (RPC) [2] of size ∼ (2 × 2 m 2 ) will be used as sensitive detectors to measure energy and direction of neutrinos.At the sea level muons are the most abundant charged particles in cosmic rays. Most muons are produced high in the atmosphere (typically at altitude of about 15 km) and lose energy of about 2 GeV via ionization before reaching the ground. The primary cosmic rays are more or less isotropic but the observed muon angular distribution at various altitudes and latitudes follows the expression I θ = I 0 cos n θ, where I 0 is the vertical integrated muon flux(cm −2 s −1 sr −1 ) and θ is the zenith angle of muon. The exponent, n and I 0 depend on momentum cut off, latitude and altitude. The experimentally observed and Monte-Carlo (GEANT4) generated theta distributions are used to extract the exponent of the cosθ distribution in data at Madurai, India. The vertical flux is estimated from the integral of the theta distribution and normalised by selection efficiency, trigger efficiency, DAQ efficiency, solid angle coverage and integrated time for data collection. All detector parameters like efficiencies, multiplicities and position smearing are obtained from observed data, which are used in the Monte-Carlo event generation. This muon flux can be used as a input of neutrino generator for ...