We have previously demonstrated that the image resolution of a small-animal positron emission tomography (PET) scanner can be improved by integrating a high-resolution insert device into the system. However, the presence of an insert device in the imaging field-of-view of a PET scanner contributes additional scatter to the data. In order to develop adequate scatter correction techniques, the quantitative accuracy of the PET images due to scatter contribution from the micro insert device needs to be carefully evaluated. The purpose of this study is to quantitatively assess the scatter fraction of a microPET F-220 scanner integrated with micro insert. The scatter fraction measurement is based on the method defined by National Electrical Manufacturers Association (NEMA) in its publication NU 4-2008. To evaluate scatter fraction contributed by the insert, a plastic tube was filled with 288.6 MBq (7.8 mCi) of 11C-sodium-bicarbonate solution and sealed at both ends before the tube was inserted into a mouse-like phantom. List-mode files were acquired repeatedly in two different energy windows. Coincidence events were sorted into sinograms and the projection profiles were processed according to NEMA NU-4 2008. Our results show that the scatter fraction of the microPET F-220 system increases by 19.3% on average with an energy window of 350-650 keV and a timing window of 6 ns when the micro insert device is attached. The performances of noise equivalent count rate in scanner and insert modes were also reported. With the spatially variant distribution of scatter fraction shown in this study, it will be critical to have an accurate correction to preserve the quantitative accuracy of PET images from the integrated system.