While enhancing the efficiency and agility to the power system, the communication infrastructure in smart grid also incurs vulnerabilities to the reliability of the power market and power grid. A novel attacking scheme is proposed, in which an attacker jams the signaling of power price and then ceases jamming when the power price has been significantly changed. The power users jammed by the attacker will make a rapid response to the changed power price, whose change direction can be predicted by the attacker. Then, the attacker can manipulate the price in the power market and thus makes profit. The abrupt change of power consumption will also cause a significant impact on the power grid stability. The effect of such an attack will be discussed both analytically and numerically. A countermeasure for this attack is also proposed.