We demonstrate sub- $100\text{-}\mathrm{as}$ timing jitter optical pulse trains generated from free-running, $77.6\mathrm{MHz}$ repetition-rate, mode-locked Er-fiber lasers. At $-0.002(\pm 0.001){\mathrm{ps}}^2$ net cavity dispersion, the rms timing jitter is $70\mathrm{as}$ ( $224\mathrm{as}$ ) integrated from $10\mathrm{kHz}$ ( $1\mathrm{kHz}$ ) to $38.8\mathrm{MHz}$ offset frequency, when measured by a $24\mathrm{as}$ resolution balanced optical cross correlator. To our knowledge, this result corresponds to the lowest rms timing jitter measured from any mode-locked fiber lasers so far. The measured result also agrees fairly well with the Namiki–Haus analytic model of quantum-limited timing jitter in stretched-pulse fiber lasers.