We have studied crustal seismic structure across the West Alborz ranges by analysis of P wave receiver functions. We used teleseismic events recorded by 24 seismic stations installed and operated by the Institute for Advanced Studies in Basic Sciences (IASBS. We used teleseismic earthquakes with magnitudes greater than 5.5 and epicentral distances between 25º and 85º. In this study, the tectonic and structural boundary between the lithosphere of the Southern Caspian Basin (SCB) and the lithosphere of the Alborz was identified. There is no evidence that the Southern Caspian Basin is underthrusting beneath the Alborz. In the coastal region, the thickness of the South Caspian crust reaches to about 35 km. implying that the SCB is unlikely to be floored by an oceanic or oceanic-like basement crust. This observation is also consistent with evidence from seismicity. The crustal thickness of central Iran in the Booin-Zahra region and further south varies between 50 to 58 km. Moho depth beneath the northern flank of the Alborz reaches to as deep as70 km. This unusually thick crust is at odds with the results of previous studies and it does not have an obvious correlation with surface geological evidence. It may be a relic of tectonic events older than the recent Arabia-Eurasia collision.