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We observed 20 Palomar-Green (PG) quasars at low redshift ($z<0.5$) withtotal flux density > 1 mJy, including 4 radio-loud quasars (RLQs) and 16radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz.Ten RQQs are clearly detected in the VLBA images, and a compact radio core isidentified in eight of them, indicating the prevalence of active galacticnucleus (AGN)-related radio emission in this flux-density-limited RQQ sample.The RQQs and RLQs in our sample have a division at $\sim$30~mJy. The radioemission from RQQs appears to be the result of a combination of star formationand AGN-associated activities. All RQQs in our sample have a 5 GHz flux densityratio of Very Large Array (VLA) A-array to D-array $f_{\rm c} = S_{\rm A}^{\rmVLA}/S_{\rm D}^{\rm VLA}$ above 0.2. The RQQs with $f_a$ (VLBA and VLA fluxdensity ratio $S^{\rm VLBA}/S_{\rm A}^{\rm VLA}) > 0.2$ versus $f_a < 0.2$ showsignificant differences in morphology, compactness and total flux density.$f_{\rm a}$ of RQQs is systematically lower than that of RLQs, probably due tothe extended jets or relic jets of RQQs on 10s to 100s parsecs which areresolved out in VLBA images. Future larger samples, especially with theaddition of milli-arcsec resolution radio images of RQQs with total fluxdensities below 1 mJy, can test the conclusions of this paper and contributesto the understanding of the radio emission mechanism of RQQs, and the dichotomyand physical connection between RQQs and RLQs.

VLBI Observations of a sample of Palomar-Green quasars I: parsec-scale morphology