A high-speed 100 MHz strain monitor using a fiber Bragg grating, an optical filter, and a mode-locked optical fiber laser has been devised, whose resolution is ΔL/L∼10−4. The strain monitor is sufficiently fast and robust for the magnetostriction measurements of materials under ultrahigh magnetic fields generated with destructive pulse magnets, where the sweep rate of the magnetic field is in the range of 10–100 T/μs. As a working example, the magnetostriction of LaCoO3 was measured at room temperature, 115 K, and 7 ∼ 4.2 K up to a maximum magnetic field of 150 T. The smooth dependence on the squared magnetic field and the first-order transition were observed at 115 K and 7 ∼ 4.2 K, respectively, reflecting the field-induced spin state evolution.