The aim of this work focuses on the study of effect of moisture on the mechanical properties of natural fiber composite (NFC) plaster/Rhecktophyllum camerunense (RC). Indeed, the treated and untreated RC fiber reinforced plaster composites are fabricated and submitted to hygrothermal ageing in an environmental enclosure of 23%, 54% and 75% relative humidities at 23°C. Flexural test was carryout under a threepoint bending approach to NF EN 6583 standard. Five specimens were tested for each case. the results indicate that the flexural properties of all the composites decreased with the increasing of moisture due to degradation of the fiber matrix interface.