Thermocycling and water storage effects on fracture toughness (
$K_\mathrm{IC}$
) of chemically or thermally tempered metal‐ceramic disks were investigated to evaluate the durability of tempering effects with regard to oral aqueous environment. Metal‐ceramic disks (
$\varnothing$
10 mm
$\times$
2.0 mm) consisting of opaque porcelain (0.2 mm thick), body porcelain (1.3 mm) and Ni‐Cr‐Be alloy (0.5 mm) were prepared and the porcelain surfaces were polished with 1‐
$\mu$
m diamond paste. The disks were subjected to ion exchange (potassium or rubidium) or thermal tempering treatments, then with the as‐polished or further annealed disks, thermocycled for 8000, 15,000, and 22,000 cycles between 5
$^\circ$
C and 60
$^\circ$
C or stored in water at 37
$^\circ$
C for 33 days. After aging,
$K_\mathrm{IC}$
values of disks were determined by a Vickers indentation technique. Statistical analyses indicated that the
$K_\mathrm{IC}$
values of ion‐exchanged and annealed disks were not significantly affected by the limited number of cycles and water storage, while the thermally tempered and untreated disks revealed a significant decrease in mean
$K_\mathrm{IC}$
from even 8000 cycles and after storage. However, there was no significant difference between thermocycling and water‐storage effects on mean
$K_\mathrm{IC}$
of all surface treatment groups. Results indicate that chemical tempering (ion exchange) produced more durable residual stresses than does thermal tempering for metal‐ceramic restorations. A low‐thermal load only was unlikely to affect the apparent fracture toughness of porcelain.
