In this paper, we discuss local unique continuation for a harmonic fun
ction on lines. By using complex extension, we prove a conditional sta
bility estimation for a harmonic function on a line. Our unique contin
uation is an intermediate property between the classical unique contin
uation for a harmonic function and the analytic continuation for a hol
omorphic function. As an application, we show conditional stability up
to the boundary ill a Cauchy problem of the Laplace equation.