  Ultra-light axion-like particle (ULAP) is one of attractive candidates for
cold dark matter. Because the de Broglie wavelength of ULAP with mass $\sim
10^{-22} {\rm eV}$ is $\mathcal{O}({\rm kpc})$, the suppression of the small
scale structure by the uncertainty principle can solve the core-cusp problem.
Frequently, ULAP is assumed to be uniformly distributed in the present
universe. In typical ULAP potentials, however, strong self-resonance at the
beginning of oscillation invokes the large fluctuations, which may cause the
formation of the dense localized object, oscillon. % Such a dense object lives
for a long time, it may affect the cosmological evolution. In this paper, we
confirm the oscillon formation in a ULAP potential by numerical simulation and
analytically derive its lifetime.
