The studies of the speckle field are extended to the area of ultrafast laser, and the concept of ultrafast speckles is introduced. In the theoretical research, the ultrafast speckle fields in the Fresnel diffraction region and the general expressions of their autocorrelation functions are discussed, and then the speckle intensity autocorrelation function is solved using the optical field autocorrelation and the intensity ensemble average. As time delays, ultrafast speckle intensity space autocorrelation curves present a characteristic of periodic oscillation and the frequency gradually increases. Combined with the computer simulation, the formation and the never-before understood dynamic behaviors of the ultrafast speckles in the Fresnel diffraction region are found and explained. Such dynamic behaviors include the increased speckle grains, the structures of the interference fringes in the speckle grains and the decrease of the spatial period, and the interference fringes gradually disappearing from two sides to the center of the observation plane, etc.