A polarization-maintaining optical fiber which exhibits an excellent polarization-maintaining performance and significantly low splice loss when spliced with a polarization-maintaining optical fiber having a cladding 125 .mu.m in diameter even if the polarization-maintaining optical fiber has a small-diameter cladding of about 80 .mu.m. The polarization-maintaining optical fiber includes a core, a pair of stress-applying parts provided radially outwardly with respect to the core, and a cladding which surrounds the core and the stress-applying parts. With a diameter D of the stress-applying parts between 21 .mu.m and 32 .mu.m, a distance R between the stress-applying parts between 6 .mu.m and 17 .mu.m, and a relative refractive index difference .DELTA. between 0.3% to 0.5%, a sufficiently large mode field diameter (MFD) is obtained; thus splice loss can be reduced.