The backdrop of this research is among others (1) the content of article 8 legislation national education system 20/2003 that the society takes the participation in planning and evaluating education, (2) the criterion of curriculum 2013 that is integrated and has a theme of natural environment, (3) the reality in the field showing that mathematics is not too meaningful, (4) do not use the school environment yet in outdoor learning so that can implant the attitude of caring about the environment. Based on the backdrop, the problem statement of this research is: how to develop thematic mathematics book based on environment with a realistic approach to implant the attitude of caring about the environment at the students of elementary school grade one in Kupang as early as possible? The method used is Research and Development with a type of Formative Research introduced by Tessmer in 1993. There are three steps in this research among others (1) the step of getting early data by using survey method analysed descriptively, (2) the step of developing the thematic mathematics books validated by the expert of education and mathematics (3) experiment step that is analysed with an inferensial statistic by using dependent t correlation analysis to know the difference of the student's mathematics knowledge before and after using the learning tool developed and to implant the attitude of caring about the environment at the students. The school used as the limited class test or small class test toward the product developed is Directive Primary School Naimata Kupang, meanwhile for the big class test is Catholic Primary School St. Arnoldus Kupang. The basic reason to choose both of the schools is the minimum of reference books used by the teachers at grade one at the schools, and they do not use the large area of the schools field as a medium in learning. The result of the research based on the problem statement showing that almost all the students is very active and have a positive response. The attitude of caring about the environment begin to appear after using the thematic mathematics book developed, as a guide book for the teachers and the practice book for the students. Besides, the students' understanding toward the number basic concept, the number operations and the two dimensions also increase and the misconception doesn't occur.