In this paper, we present an effective method to enhance visual reasoning
with significantly fewer training samples, relying purely on self-improvement
with no knowledge distillation. Our key insight is that the difficulty of
training data during reinforcement fine-tuning (RFT) is critical. Appropriately
challenging samples can substantially boost reasoning capabilities even when
the dataset is small. Despite being intuitive, the main challenge remains in
accurately quantifying sample difficulty to enable effective data filtering. To
this end, we propose a novel way of repurposing Monte Carlo Tree Search (MCTS)
to achieve that. Starting from our curated 70k open-source training samples, we
introduce an MCTS-based selection method that quantifies sample difficulty
based on the number of iterations required by the VLMs to solve each problem.
This explicit step-by-step reasoning in MCTS enforces the model to think longer
and better identifies samples that are genuinely challenging. We filter and
retain 11k samples to perform RFT on Qwen2.5-VL-7B-Instruct, resulting in our
final model, ThinkLite-VL. Evaluation results on eight benchmarks show that
ThinkLite-VL improves the average performance of Qwen2.5-VL-7B-Instruct by 7%,
using only 11k training samples with no knowledge distillation. This
significantly outperforms all existing 7B-level reasoning VLMs, and our fairly
comparable baselines that use classic selection methods such as accuracy-based
filtering. Notably, on MathVista, ThinkLite-VL-7B achieves the SoTA accuracy of
75.1, surpassing Qwen2.5-VL-72B, GPT-4o, and O1. Our code, data, and model are
available at https://github.com/si0wang/ThinkLite-VL.