Despite a significant surge in the number of investigations into chirality at the nanoscale, especially thiolated chiral molecules capping gold clusters, only limited knowledge is currently available to elaborate the alloying effect on chiroptical behavior of bimetallic nanoparticles (NPs). Also, few successful cases as to the efforts toward the development of chirality-dependent applications on the optically active nanomaterial have been made. Herein, as a positive test case for chiral alloy nanoparticle synthesis, the stable and large chiroptical ultrafine Au-Ag alloy NPs were prepared by reduction of different molar fractions of HAuCl4 and AgNO3 with NaBH4 in the presence of d/l-penicillamine (d/l-Pen). Compared with those of monometallic Au and Ag counterparts with comparable size, the Au-Ag alloy NPs (Ag mole fraction, 70%) obviously displayed the largest optical activities with the maximum g-factors of ∼1.6 × 10-3. Impressively, the Pen-mediated synthesis of chiral Au-Ag alloy NPs possesses a colorimetric self-recognition function and can be used as an incisive circular dichroism (CD) probe toward d- and l-Pen enantiomers. The plasmonic CD signal amplification (ΔICD) shows good linearity with the amount of Pen over the range of 5.0-80.0 μM with a detection limit (3σ) of 1.7 μM for l-Pen and 1.5 μM for d-Pen, respectively. In addition, the sensing system exhibits good selectivity toward d- and l-Pen in the presence of other enantiomers; therefore, it is highly expected that the approach described here would open new opportunities for design of more novel enantioselective analyses of important species related to biological processes.