Abstract: Quantitative analysis for angular oscillation is conducted,from the observations of pitch angleθand yaw angleΨof an aircraft in two flight tests.Aerodynamic parameter identifica-tion for Van der Pol equation θ··= C00θ+Cm1 (1-C11θ2 )θ· and ·Ψ·= C00Ψ+Cn1 (1-C22Ψ2 )Ψ· de-scribing angular oscillations in pitch and yaw planes respectively,and parameter fitting for the outer envelope curve ofθ-t and Ψ-tobservations,are both carried out.The fitting and parameter identification curves of two flight tests both show good agreement with observations,which means the fitting and parameter identification results are both reliable.All the parameters,inclu-ding Cm1 ,C11 ,Cn1 and C22 ,obtained from parameter identification are similar to those from fit-ting of outside envelope curves in the two flight tests,except a bit difference for C11 by the two methods for the first flight test.The conclusion can be made that the angular oscillations in pitch and yaw planes are the typical non-linear negative damping limit cycle type oscillation,for Cm1 ,C11 ,Cn1 and C22 are all positive while C00 is negative.And that indicates the coning oscillation of the aircraft is stimulated by the aerodynamic damping moment of non-linear negative damping limit cycle type.The typical expression of the aerodynamic damping moment of non-linear nega-tive damping limit cycle type can be acquired according to parameters above.Finally,it is verified that dynamic instability of aircrafts can be brought by the aerodynamic damping moment of non-linear negative damping limit cycle type.