Our aim is to study the effect of pen pal programs on child development. We conduct a randomized controlled trail on students in grade 7 (first-year middle school students) in China, in collaboration with the BlueLetter, an NGO specialized in organizing pen pal programs for many years. The program matches each student in the treatment group with a pen pal volunteer, and allow them to communicate by mail for one and half years anonymously and under the supervision of BlueLetter. We plan to (1) examine the causal effect of pen pals on a range of development outcomes, including test scores, cognitive skills, and noncognitive skills (Big Five personlity factors) as well as economic preferences, both immediately after the program ended and longer term effects; (2) study heterogeneous effects for different subgroups. In particular, we will focus on left behind children whose parents were away from home for work in cities. Left behind students are especially likely to suffer from emotional issues, thus could potentially benefit from pen pal programs which provides emotional support; (3) analyze the underlying mechanisms through which the program impacts children. We have collected extensive information regarding the children, their parents and schools before the intervention, and will do so after the RCT. Meanwhile, we will analyze the text contents of the letters exchanged between the student and pen pal volunteers.