When a fixed free-running crystal is used for sampling time generation at a DMT receiver, Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI) are introduced by sampling time error. The ICI becomes more serious as the DMT symbol length increases. In this letter, the effects of sampling clock offset are investigated in the time domain using a new notion of Inter-sample Interference (IsI) instead of ISI and ICI. Based on the Isl analysis, we propose a new frequency domain timing error correction scheme.