Bio-based composites are presented as ecological alternatives to synthetic materials whose production contributes to the depletion of natural resources and climate change. This study aims to develop a new material from cotton stem particles (CSP) and Grewia Venusta bark powder (GWP) intended for the manufacture of thermal insulation panels. Eleven (11) formulations were selected by varying the CSP rate and the granular classes of CSP under a temperature of 140°C. This work specifically aims to physically and mechanically characterize this material with the European standards EN 310, 322 and 323. The results obtained show that the density of the composites increases from 402.08 to 998.69 Kg/m3 when the binder rate is increasing, and decreases from 895.35 to 793.38 Kg/m3 when the granular class of the particles increases. The Elastic Flexural Modulus MOE) and the Flexural Breaking Strength (MOR) increase respectively from 162.25 to 345.03 MPa and from 4.53 to 5.53 MPa for a binder rate varying from 60 to 75%, then decreases when this rate takes a value above 75%. These values increase from 209.82 to 562.28 MPa and from 5.65 to 6.18 MPa, respectively, as the grain size increases from [0.063-0.630 mm] to [0.063-2 mm]. The formulation conforming to the required ANSI 208.1.1999 standard has a density of 691.15 kg/m3, an MOE of 562.28 MPa, and an MOR of 6.18 MPa.