  Quantum entanglement is an indispensable resource for many significant
quantum information processing tasks. Thus, distilling more entanglement from
less entangled resource is a task of practical significance and has been
investigated for decades. The literature [Verstraete \textit{et al}.,
\href{https://link.aps.org/doi/10.1103/PhysRevA.64.010101}{Phys. Rev. A 64,
010101(2001)}] considered a scenario to increase the entanglement by local
filtering operation and qualitatively derived the variance relation of
entanglement. We investigate the scenario with general two-qubit resources to
find the optimal strategy of filtering operations. We obtain the upper bound
for the ratio of entanglement increase and find the corresponding optimal local
filtering operation to achieve the maximal ratio. Our analysis shows that the
upper bound ratio grows with the length of local Bloch vector while the success
probability decrease with it. We further extend the research to investigate the
optimal measurement strategy by considering general measurement. Our result
shows that local measurement can not increase the expectation of quantum
entanglement, which gives more analytical evidence to the well known fact that
local operation can not create quantum entanglement.
