A new q uantum s ta t is t ica l (inharmonic theory has been derived fo r cal culation and analysis o f XAFS cumulants and thermal earpansion o f bcc A B binary alloys systems. This model is developed based on the anharmonic vibration o f ab sorber from atom sorte A and backs catterer from another atom sorte 11, including contributions o f th e ir nearest neighbours. Atomic pair potential has been taken by plus-averaging o f Morse potential. The expressions have been derived fo r effective spring constant, correlated E in s te in frcqiicncy, correlated E in s te in temperature, f ir s t cumulant or ne t therm al expansion, second cumulant or Debye- Waller factor , third cumulant, causing phase chanqe o f XA F S spcct.ro, and thermal expansion coefficient. Numurical evaluations have been carried out fo r F eI _ s Wx. The results are temper ature dependent and Ttfleet tJic experiment and fundamental theoretical bchaviuors o f these quantities.