Main minerals of rocks are all polar materials. The polarity of formation water is stronger than that of oil, so underground rocks are water-wet and there are no oil-wet rocks underground. The water-wettability of rocks provides the precondition of migration of oil and gas from source rock to trap. Oil-wet rocks will disperse the oil and gas during migration, and can't allow the hydrocarbon accumulation to be eompleled at all. Relative permeability curves measured at laboratory indicate the water-wettability of rocks clearly. The normal distribu-tiun of reservoir pressure is also an evidence of water-wettablity of underground rocks. Oil-wet rocks won't prohibit the migration of oil and gas as a seal rock but allow oil and gas to he dispersed through them. The oil-wettability of rocks measured at laboratory does not reflect the underground conditions, so it is meaningless.