While there has been great effort to establish universal behavior of the se
quence of period-doubling bifurcation in Hamiltonian systems with few degre
es of freedom, the nature of the period-doubling bifurcation is far more co
mplicated in two-dimensional maps. Though the onset of instability is deter
mined by a local, linear property of the system, the area of a bifurcated r
egion in the phase space increases gradually when the control parameter inc
reases beyond the critical threshold. Scaling laws for the growth process o
f the period-doubling bifurcation are elucidated for the period-2 step-1 ac
celerator mode and for the fundamental fixed orbit in the standard map.