The good pursuance of plate needs continuous measurements and analysis under working circumstances to avoid the unusual loading or occurring sudden stresses. Pursuance of the mechanical behaviour of composite plates by pursuing the fundamental natural frequency of the plates, gives an index of existence of a defect in the plate occurred as depreciation. The defect may be as a crack on the surface or inner crack or any type of defects that may be caused by mechanical or thermal applied loads. The existence of a defect like a crack which will lead to change in fundamental natural frequency of the plate and enlargement of the crack will also lead to another change in fundamental natural frequency with the change of the size or position of the crack. So this study focuses on finding the fundamental natural frequency for composite plates with crack, considering the size of the crack (crack length and depth through plate thickness) and crack position in the plate in x and y directions, as well as slant of the crack. The study of the fundamental natural frequency of the composite plate is done on many types of composite plates, according to the shape of the used fibers. So this involves a study of properties and types of composite materials. The study also handles the desperation of composite material containing particles, powder, short fibers, long fibers and mat, which an strengthened by particles and short fibers which give isotropic materials and strengthening by long fibers and mat gives orthotropic material. The composite materials are tested to find their properties experimentally by preparing patterns and changing volume fraction of fibers and matrix analytically. A comparison is made between the analytical and experimental results where the error percentage was found not to exceed 8%. The fundamental natural frequency is studied for composite material strengthened by particles, powder, short fibers, long fibers and mat, in terms of the effect of crack size and position, plate thickness, aspect ratio and the type of plate fixing where three types of fixing were used (SSSS, SSCC, SSFF). Three methods are used to find the fundamental natural frequency of composite plate: First method is analytical solution to solve the equation of motion considering the effect of size, position and crack slope angle on the fundamental natural frequency of the composite plate. Second method is finite element solution using ANSYS (ver. 14) program. Third method is an experimental method, applying a time varied load to measure the fundamental natural frequency of the plate. A comparison is made between the three methods and the error percentage in the results does not exceed 8.5%. in addition to comparison between the present theoretical and numerical results with results presented by S. E. Khadem and M. Rezaee-a (2000), where the error not exceed 4%. The results show that the fundamental natural frequency decreases as crack size (length or width) increases to (5%) with crack length (10%a) to (8%) with crack length (20%a) and crack depth ratio (0.7). The fundamental natural frequency decreases when the crack is in the middle of the plate larger than from any position in the plate, decreasing of the natural frequency to about 6%. The effect of crack when reaches the middle is higher than when it is in the other places. The fundamental natural frequency decreases as plate width increases, (aspect ratio and plate thickness).