A lock detector for a phase lock loop (PLL) includes: first and second pulse width extenders, performing pulse width extension on first and second pulses for generating third and fourth pulses, respectively; first and second delay circuits, delaying the third and the fourth pulses into first and second sampling clocks, respectively; and a cross-sampling circuit, sampling the third pulse based on the second sampling clock and sampling the fourth pulse based on the first sampling clock to indicate whether the PLL is locked.