Ultrafast electron diffraction （UED） is a powerful technique for the measurement of transient structures. Employing our UED system, with its capability of detecting a sub-milli-ngstrom lattice spacing change with a sub-picosecond temporal resolution, we performed the real-time observation of both the coherent phonon and thermal lattice motions of polycrystalline 20 nm Al film, which was excited by femtosecond laser pulses. The obtained coherent oscillation period of ～7.4 ps and temperature rise of ～77 K agree well with those from theoretical calculation. This achievement offers great promise to conduct measurements of transient structures with atomic spatiotemporal resolution in many dynamical processes that can be initiated by ultrafast optical excitation.