In this Letter, we report the observational constraints on the Hu-Sawicki f(R) theory derived from weak lensing peak abundances, which are closely related to the mass function of massive halos. In comparison with studies using optical or x-ray clusters of galaxies, weak lensing peak analyses have the advantages of not relying on mass-baryonic observable calibrations. With observations from the Canada-France-Hawaii-Telescope Lensing Survey, our peak analyses give rise to a tight constraint on

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... model parameter |f_R0| for n=1. The 95% CL limit is _10|f_R0| < -4.82 given WMAP9 priors on (Ω_ m, A_ s). With Planck15 priors, the corresponding result is _10|f_R0| < -5.16.