Composites are an essential part of today’s materials because they offer advantages such as low weight, corrosion resistance, high fatigue strength, faster assembly, etc. Composites are used as materials ranging from making aircraft structures to golf clubs, electronic packaging to medical equipment, and space vehicles to home building. Composites are generating curiosity and interest in students all over the world. This book describes the systematic study on composite material and R.V.E (Representative Volume Element). Then Simulation occurred on cotton/epoxy unidirectional composites with FEA of TPREV (Three Phase Representative Volume Element) considering interphase layer as third phase and by changing the volume fraction of fiber and corresponding at different volume fraction also changing the geometry of fiber using ANSYS APDL language and as per the matrix failure criteria choose the experimental displacement of epoxy resin through simulation and found out the tensile strength of composites. Finally compare the analytical results through FEA simulation with experimental for cotton/epoxy composites and predict tensile strength.