Optical depth is significant key vegetation parameters in the study of vegetation canopy. Until now efforts have been made to estimate optical depth by fitting sensors brightness temperature to a zero-order model (τ-ω model). The τ-ω model is a simple model without considering multi-scattering effects inside the vegetation layer. Here effective optical depth (VODs) calculated from a high order model to take in to account multi-scattering effects. The high order model is a close simulation of the real situation. It enables investigating optical depth behavior during wheat grow season in different frequencies, incident angels and polarizations. Measured LAI, VWC and biomass were used to validate simulated VODs. VODs was corresponding to the measured vegetation parameters in terms of the trend and the time of maximum and the minimum values. Furthermore, equivalent optical depth (EVOD) was calculated by fitting brightness temperature to the zero-order model and compared with the VODs. Results indicated EVOD as VODs also were frequency, incident angle and polarization dependent but overestimated compare to VODs. Calculating optical depth of other types of vegetation with different structure can give more detailed in vegetation behavior during growing season.