Thermal expansion coefficient of YBa2Cu3O7-x with the tetragonal and orthorhombic structural phases were measured by the method of differential parallel-plate capacitance. There exist anomalies near 205 K for orthorhombic phase and between 92 K-130 K for tetragonal phase respectively. We suggest that the former may be related to the lattice instability, indicating that there is a strong electronphonon interaction; the latter may result from the structure phase transition which destroys the high Tc superconductivity. A small jump of the thermal expansion coefficient at Tc was observed in the orthorhombic phase and from this the pressure effect was estimated.