This work proposes a forest model based on Gamma distribution to better describe the forest vertical structure for height inversion using P-band polarimetric synthetic aperture radar interferometry (Pol-InSAR) data. The proposed model takes into account the forest vertical heterogeneity and asymmetry, to which volume interferometric coherence is sensitive. The interferometric coherence associated with a volume where the vertical backscattered power varies following a Gamma distribution is derived. The effect of scattering center height standard deviation and mean elevation to the volume interferometric coherence is investigated. Finally, the strategy of multi-baseline on the proposed model for forest height inversion using P-band Pol-InSAR data is proposed.