Objective of the study lies on assisting the professional growth of two practicing teachers through the help of Mathematics Teaching Development Group in one elementary school. Case studies have been conducted at Taipei County on elementary schools in the urban district of Hsin-tien. Information on group meeting analyses, teaching observations, and collections of relevant documents have been gathered to savvy the undergoing-progress of practicing teachers and their reflections on Mathematics teaching since joining the growth group. Influences of the growth group on the professional development of practicing teachers on Mathematics teaching are annotated. Two significant conclusions are derived. Firstly, the group focuses on the empirical teaching of Mathematics rather than on theoretical discussions. The developmental activities done by the growth group can further the Mathematics teaching efficacy of practicing teachers. Secondly, reflections of practicing teachers on Mathematics teaching differ in “quantity” and “quality”; thereby enhancing concerns of teaching by confabulating the empirical teaching of Mathematics; elucidating the relationship between teaching belief and empirical teaching emanated from the activities of Mathematics professional development; maintaining the professional development dynamism through supportive surroundings.