Tho synchronizstion of sn S.bantl Trapatt oscillator working in the fundamental made hns been investignted. The phshift batwesn the input and output signal has bean menaurorl in dependence upon tho frcquenoy deviation from the freerunning oscillator for various input power levels. I t is shown that the phase differ. ence can be described by Adler's looking oquntion using s new defined figum of merit. The high injection signols required for stable lacking together u.ith the significant non.linemity oi the T r a p t t diode cnuso a n output power variation, wldch diiiers from the conventionel locking ellipses. As a new result, the charac. taristic reduction a i tho output powor within tho locking rango is explained by a model using a constant current source for the Trapatt diode. The theoretical reaulte are in good agreement with measurements.