R1-Code-Interpreter is the first framework to train LLMs for step-by-step code reasoning using multi-turn supervised fine-tuning and reinforcement learning. By curating 144 diverse reasoning and planning tasks, we enable Qwen-2.5 models (3B/7B/14B) to autonomously decide when and how to invoke code. Our best model, R1-CI-14B, outperforms GPT-4o (text-only) and approaches GPT-4o with Code Interpreter, showing emergent self-checking behavior via code generation.