This paper studies on microstrip leaky-wave antennas (MLWAs) with longitudinally uniform and nonuniform periodical loadings of shorting pins. The complex propagation constants of uniform and non-uniform periodical loadings on both fundamental EHo and higher-order EHi mode of the microstrip line are numerically extracted and analyzed. Thus, unique features such as arbitrarily-determined operating frequency regions and flexibly-controlled fast-wave bandwidths are explicitly demonstrated. Two antenna prototypes including uniformly-loaded EH0-mode MLWA and non-uniformly-loaded EH1-mode MLWA are designed, implemented and measured. Tested results verify the attractive advantages of uniform and non-uniform pin-loaded MLWAs, exhibiting great potentials of our pin-loaded technique to be applied into MLWAs with advanced functionalities.