The ideal length of study for undergraduate students is about four to five years, but the fact is there are still many students of FMIPA Unsyiah takeslonger time to graduate. This research aims to classify the factors that affect that lenght of study based on the provided data in FMIPA Unsyiah, which are some data of characteristics and educational background of the students. The statistical methods used are CHAID (Chi-Squared Automatic Interaction Detection). CHAID method will divide the data into groups of smaller based linkages between response variable with the explanatory variables.This research used the data from undergraduate students who have graduated from the period of January 2007 to August 2014. It is found that the significant factors influencing the students’ lenght of study are the GPA, the education level of supervisor, the final exam score, the gender, and the program of study.value of the session. It is also found that the students whose lenght of study in time and average are students with excellent GPA score, received grade A in their final exam, and are female. While students within late lenght of study category are students in satisfactory GPA with the level education of their final writing supervisors are doctorate. In terms of classification accuracy, the percentage of accuracy ofthe model is 61.4%.