The aim of this study was to determine the influences of monopropylene glycol (MPG) and oleic acid (OA) as novel additives on the chord length distribution (CLD) and modification of the shape of boric acid crystals in real time with the use of in situ focused beam reflectance measurement (FBRM) and particle vision and measurement (PVM) probes, which were positioned on a continuous mixed-suspension mixed-product removal (CMSMPR) crystallizer at a steady-state value. In this context, the FBRM probe was used to monitor CLD, which is expressed as the cumulative undersize square-weight percentage distribution of boric acid crystals. CLD is statistically proportional to crystal size distribution. The shapes of the boric acid crystals were viewed in real time with an in situ PVM probe. In addition, a scanning electron microscope (SEM) and an optical microscope were used to characterize the shapes of the boric acid crystals produced in the CMSMPR crystallizer. The chemical structures of the boric acid crystals were characterized by Fourier transform infrared (FT-IR) analysis. The population density of the nuclei, the nucleation rate, and the growth rate of the boric acid crystals were also calculated. As a result, it was proved that industrial problems encountered in the production of boric acid crystals by the crystallization process were eliminated especially in the presence of 100 ppm OA and 50 ppm MPG additives by FBRM, PVM, SEM analysis, and number density theory application. This study revealed novel insights into the modification of the shape of boric acid crystals and into the control of CLD in the presence of OA and MPG using in situ FBRM and PVM probes.