We have examined the electrical and optical characteristics of the plasma display panel produced by vacuum in-line sealing technology. In the MgO layer deposited at room temperature, after sealing at the panel temperature of $430^{circ}C$, the luminous efficiency decreased compared with that of the panel before sealing. Moreover, firing and sustain voltage of the sealed panel increased compared with that of the panel before sealing. This was resulted from that the MgO protective layer was cracked by the softening of the dielectric layer during the sealing process. In order to avoid the MgO crack during the vacuum in-line sealing, thermally stable MgO layer or lower temperature sealing is required.