This paper describes a controlled experiment of student programmers performing maintenance tasks on a C++ program. The goal of the study is to assess the maintenance size, effort, and effort distribution of three different maintenance types and to describe estimation models to predict the programmer’s effort on maintenance tasks. The results of our study suggest that corrective maintenance is much less productive than enhancive and reductive maintenance. Our study also confirms the previous results which conclude that corrective and reductive maintenance requires large proportions of effort on program comprehension activity. Moreover, the best effort model we obtained from fitting the experiment data can estimate the time of 79% of the programmers with the error of 30% or less.