Measurements of microwave surface resistance, R s , at subcritical currents as a function of temperature with varying dc magnetic field upto 0.8 T have shown peak effect (PE) in epitaxial DyBa 2 Cu 3 O 7รd (DBCO) and YBa 2 Cu 3 O 7รd (YBCO) thin films grown by pulsed laser deposition on AE1 0 0ae LaAlO 3 substrates. Microwave measurements were performed on microstrip resonators as test vehicles. Occurrence of a peak in R s in dc magnetic field is governed by the nature and concentration of defects. Evidence shows that thinner films with a higher ratio of areal density of extended defects, n e (such as twin boundaries), to the areal density of point defects, n p , show PE at the measurement frequencies 4.88 and 9.55 GHz; whereas, thicker films (P3000 A) with a smaller n e =n p ratio do not show PE. 2500 A thick YBCO film shows a double peak structure at 9.55 GHz, thereby suggesting two sets of twin boundaries in this film having different j p values. Measurements carried out on low-twinned LaAlO 3 substrates show that 2400
A thick DBCO film does not exhibit the PE phenomenon at 4.88 GHz upto to an applied field of 0.8 T; this indicates that twins propagated from the LaAlO 3 substrates are responsible for the occurrence of PE at microwave frequencies. Oxygen ion irradiation (90 MeV, 3 โข 10 13 ions/cm 2 ) of 2500
A DBCO film has been found to shift the peak to lower temperature at 4.88 GHz, but significantly suppress the peak at 9.55 GHz. Depinning frequency, x p vs. T plot obtained for the 2400 A DBCO film shows a peak due to the peak in its R s vs. T plots.