Abstract: In this paper,a new algorithm for high accuracy deformation monitor with GPS is proposed.Double difference observations Δ▽φ'₀ are formed using the carrier phase observations at monitor point and reference station.The calculated double difference observations Δ▽φ_c are derived from the prior known position of the monitor point and reference station,then the deformation vector δμ can be derived from the difference between Δ▽φ_c and Δ▽φ'₀ if the deformation value is very small,and its effect on δΔ▽φ is far less than half one cycle.Cycle slip detection and repair and ambiguity resolution can be avoided in the data process using the proposed method.The data processing procedure can be simplified greatly.The performance of the proposed method has been tested using data from a baseline within about 1 km.The preliminary result indicates that the new algorithm and the mathematic models are correct,and shows that the deformation estimates are at the accuracy of several millimeters with two hours data.A software package was developed to implement the proposed methodology.