The evolution of oxide inclusions during isothermal heating of 18Cr–8Ni stainless steel with yttrium addition at temperatures of 1273 to 1573 K was investigated systematically. Homogeneous spherical Al–Y–Si(–Mn–Cr) oxide inclusions were observed in as-cast steel. After heating, most of the homogeneous inclusions were transformed into heterogeneous inclusions with Y-rich and Al-rich parts, even though some homogeneous oxide particles were still observed at 1273 and 1573 K. With the increase in heating temperature, more large-sized inclu-sions were formed. The shape of the inclusions also changed from spherical to irregular. The maximum transformation temperature of inclu-sions was determined to be 1373 K. The evolution mechanism of inclusions during heating was proposed to be the combined effect of the (i) intal transformation of inclusions due to the crystallization of glassy oxide and (ii) interfacial reaction between inclusions and steel matrix. Meanwhile, the intal transformation of inclusions was considered to be the main factor at heating temperatures less than 1473 K.