Structural changes in Y1-xCaxTiO3 (x=0.37, 0.39 and 0.41) during a transition from an insulator-to-metal were investigated by transmission electron microscopy. In a specimen for x=0.39, on cooling a transition from orthorhombic to monoclinic lattice was observed first. The onset of this transition was not certain, but the monoclinic lattice was clearly observed at 170 K. On further cooling to 140 K, approximately the metalinsulator transition temperature, an introduction of strain to this monoclinic phase was observed. Below 140 K a state of phase separation, with growth of domains, was observed. In the insulator specimen for x=0.37 the transition from orthorhombic to monoclinic lattice was observed similarly to the case of x=0.39, but no phase separation was observed in the range from room temperature to 110 K. In the case for x=0.41, the phase separation was also observed on cooling, but transition temperature of the phase separation is lower than that reported by x-ray diffraction. The phase separation observed presently for 0.39 and 0.41 is considered to be a kind of electrical phase separation, which drives the appearance of the metallic state at low temperatures. This is the first report on observation of separated domains on the electrical phase separation associated with the metal-insulator transition in paramagnetic Y1-xCaxTiO3.