The effect of calcium ions on the adsorption of sodium oleate on the mineral surfaces and flotation separation of cassiterite from quartz were studied in this work through micro-flotation tests, zeta potential measurements, solution chemistry calculation, surface adsorption experiments, and X-ray photoelectron spectroscopy (XPS) analysis. The micro-flotation tests showed that calcium species had little effect on the floatability of cassiterite and quartz at acidic pH, but the effect significantly changed at pH higher than 9.1. Cassiterite had good float ability at pH 8.2 and was less affected by calcium species. Quartz exhibited poor floatability in the absence and presence of calcium ions around pH 8.2, which facilitated its flotation separation from cassiterite. Zeta potential and surface adsorption results indicated that the adsorption of sodium oleate on the mineral surface was minimally influenced by calcium ions at acidic pH but was greatly affected at strong alkaline pH because of the specific adsorption of hydrated Ca(OH)(+) and Ca(OH)(2) on the mineral surface. The adsorption mechanism was identified by XPS analysis, demonstrating that calcium species were adsorbed onto the mineral surface through the interaction with O sites of cassiterite and quartz particles.