This book elements of Nuclear reactor has A been written to meet the requirement of the Student of pass course, honours and post graduate students the subject matter of this book is presented in very straightforward matter. Nuclear reactor is a very important part of nuclear physics, having a broad field, we have try to maintain this field under the small volume according our best adherence. Matter of this book is explained in very simple grasping way with a suggestion of lecturer`s authors and post graduate students. This book is very helpful of the students studying in graduation and post graduation level and the candidate appearing in various competitions like I.A.S. P.C.S. etc. Contents, 1. Interactions of Nuclear Radiation with Matter 2. Nuclear Reactions 3. Nuclear Models 4. Neutrons