Abstract: 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 completed at all. Relative permeability curves measured at laboratory indicate the water?wettability of rocks clearly. The normal distribution 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 be dispersed through them. The oil?wettability of rocks measured at laboratory does not reflect the underground conditions, so it is meaningless