Lack of non-repudiation is a major barrier of building a trustworthy smart grid. In current power systems, bills are generated based on the amount of service consumed by residential or commercial users. However, meter readings may not be trustworthy due to malicious behavior (e.g., energy theft) or external attacks. The root cause is that power providers have no means to obtain the reading value other than receiving it from the users. To resolve this issue, we present a mutual inspection strategy, which enables nonrepudiation on meter readings for smart grid. The goal of our scheme is to discover problematic meters that report inaccurate reading values.