Series of bulk and nanostructured La_(0.7)Sr_xCa_(0.3-x)MnO_3(x = 0.10, 0.15, and 0.20) manganites were synthesized and characterized using different techniques. In the series, both the bulk and nanostructured La_(0.7)Sr_(0.10)Ca_(0.20)MnO_3 and La_(0.7)Sr_(0.20)Ca_(0.10)MnO_3 manganites have orthorhombic and rhombohedral structures, whereas La_(0.7)Sr_(0.15)Ca_(0.15)MnO_3 manganite has rhombohedral and orthorhombic structures, respectively. Online ultrasonic velocity and attenuation measurements were taken through an indigenously designed ultrasonic setup, and the analysis was done from 300 to 400 K during the aging of the samples to explore the structural/phase transitions. The bulk and nanocrystalline La_(0.7)Sr_xCa_(0.3-x)MnO_3 perovskite samples show the particle size distribution in the range of 197–943 and 24–93 nm, respectively. The addition of Sr~(2+) alters the size of particles, which decrease in size. The observed anomaly in ultrasonic velocities, attenuations, and elastic moduli is correlated with the ferromagnetic–paramagnetic(FM–PM) transition temperature(T_C) in both bulk and nanocrystalline perovskites. In addition, the shift in T_C and the magnitude and width of observed anomaly are correlated with the value of x to study the behavior of T_C. The series, bulk and nanostructured La_ (0.7) Sr_xCa_ (0.3-x) MnO_3 (x = 0.10, 0.15, and 0.20) manganites were synthesized and characterized using different techniques. (0.2) Mn0_3 and La_ (0.7) Sr_ (0.20) Ca_ (0.10) MnO_3 manganites have orthorhombic and rhombohedral structures, respectively, while La_ (0.7) Sr_ (0.15) Ca_ (0.15) MnO_3 manganites have rhombohedral and orthorhombic structures . Online ultrasonic velocity and attenuation measurements were taken through an indigenously designed ultrasonic setup, and the analysis was done from 300 to 400 K during the aging of the samples to explore the structural / phase transitions. The bulk and nanocrystalline La_ (0.7) Sr_xCa_ 0.3-x) MnO_3 perovskite samples show the particle size distribution in the range of 197-943 and 24-93 nm, respectively. The addition of Sr ~ (2+) alters the size of particles, which decrease in size. The observed anomaly in ultrasonic velocities, attenuations, and elas tic moduli is correlated with the ferromagnetic-paramagnetic (FM-PM) transition temperature (T_C) in both bulk and nanocrystalline perovskites. In addition, the shift in T_C and the magnitude and width of observed anomaly are correlated with the value of x to study the behavior of T_C.