My late father, Professor Young Chung-Chien, had a good habit during his lifetime, that many letters to him had been well preserved. Here Ⅰ would like to introduce three letters that Professor Li Siguang (J.S.Lee) had written to him sixty-four year ago, these letters related to the creation of vertebrate paleontology in China. Professor Li Siguang, not only a pioneer scientist but also an enthusiastic educationist as well, was a professor at the Department of Geology of Peking University (Beijing) in the 1920's, and my father was one of the initial students of him. In his memoir he had written that "Mr. Li remained led us go on a field trip while teachers were on strike, his spirit of hard work has absolute driving force to me". Under Professor Li's encouragement, my father had gone to Germany to study vertebrate paleontology, in 1927, his dissertation entitled "Fossile Nagetiere aus Nord-China" was published and this may be considered as the beginning of vertebrate paleontology researches undertaken by Chinese scientists. This paper has made some annotations on the three letters, from the encouragement like this up to the concrete arrangements by Professor Li, we may clearly appreciate how sincere a friendship, that based on common devotion to science, it is. On the occasion of the centenary of the birth of Professor Li Siguang, Ⅰ think,perhaps, it is also a commemorative form to review the historicals of the friendship of over fifty years between the two scientists.