We have examined the crystal structures of heavy-fermion compounds Ce3Al and Ce3Sn at low temperatures by neutron diffraction. The Ce3Al compound retains the monoclinic structure (the gamma-phase), with the angle between the a and b axes is very close to 90-degrees, down to the lowest temperature studied of T = 0.35 K. This crystal structure Of Ce3Al at low temperatures is fairly complicated, which contains sixteen atoms in the unit cell, and is originated from a small distortion from the hexagonal structure (the alpha-phase) found at higher temperatures. Although two structural phase transitions have already been observed in Ce3Al, our results show that no structural phase transition is involved in the magnetic phase transition. On the other hand, the diffraction pattern taken at T = 1.8 K on the Ce3Sn compound shows that it retains the fcc structure found at high temperatures. In which the Sn atoms located at the corners of the cubic unit cell and the Ce atoms at the center of each face of the cube. This crystal structure of Ce3Sn is relatively simple, which contains only four atoms in the unit cell.