Provided is an ultrafine polyester fiber which is useful as a constituent material for a cloth for a stent graft or other medical device that is to be implanted in the body and which can simultaneously solve both a clinical need (diameter reduction) and a clinical challenge (integration of a stent with a stent graft). An ultrafine polyester fiber which has a polyethylene terephthalate content of 98 wt% or more, characterized by: having (1) a reduced viscosity (ηsp/c) of 0.80 dl/g or higher and (2) a total fineness of 7 to 120 dtex and a single-fiber fineness of 0.5 dtex or less； and exhibiting (3) a maximum thermal shrinkage stress of 0.05 cN/dtex or more in a temperature range of 80 to 200°C or (4) a degree of crystallinity of 35% or more in a region spreading from the surface of the fiber to a depth of 0.1 µm.
