The strain energy release rate
G
required to propagate a crack at velocity
V
along an infinite viscoelastic strip of width 2
h
his calculated numerically. It is shown that
G
can be approximated by 2
γC
(
h/V
)/
C
(
l/V
), where
γ
is the intrinsic fracture energy,
C
(
t
) is the creep compliance function, and
l
is the Barenblatt or Dugdale zone length. The analysis predicts a region of the
GV
curve having negative gradient, providing a possible explanation for the instability phenomena observed in the fracture of polymers. The critical point (
G
c
,
V
c
) at which d
G
/d
V
= 0 , is dependent on specimen size, with large specimens having greater apparent fracture surface energy than small ones. When applied to polymethyl methacrylate (PMMA), the predicted value of
V
c
is in close agreement with the velocity observed at the transition from slow to fast crack growth.
